fix: Update mongodbatlas_cloud_backup_schedule to add the ID field to policyItems

[andreaangiolillo]

Description

Ticket: INTMDB-970

This bug was discovered as part of HELP-4841: currently mongodbatlas_cloud_backup_schedule is updating backup policies without providing the id fields for policyItems. This prevents atlas from updating old snaphosts to use the new policy.

This PR updates the logic to include the ID field to policyItems` when updating the resource.

Type of change:

• Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR.
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the contribution guidelines
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code

Further comments

Testing

Updated acceptance test

Running tool: /Users/andrea.angiolillo/.asdf/shims/go test -timeout 300000s -run ^TestAccBackupRSCloudBackupSchedule_basic$ github.com/mongodb/terraform-provider-mongodbatlas/mongodbatlas

ok  	github.com/mongodb/terraform-provider-mongodbatlas/mongodbatlas	772.266s

Tested locally and checked that the policy retemption date was updated

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  ~ update in-place

Terraform will perform the following actions:

  # mongodbatlas_cloud_backup_schedule.test will be updated in-place
  ~ resource "mongodbatlas_cloud_backup_schedule" "test" {
        id                                       = "Y2x1c3Rlcl9uYW1l:Q2x1c3RlcjA=-cHJvamVjdF9pZA==:NjNmNGQ0YTQ3YmFlYWM1OTQwNmRjMTMx"
        # (11 unchanged attributes hidden)

      ~ policy_item_hourly {
            id                 = "64c8e99dcca1e53ba52324b6"
          ~ retention_value    = 5 -> 6
            # (3 unchanged attributes hidden)
        }

        # (2 unchanged blocks hidden)
    }

Plan: 0 to add, 1 to change, 0 to destroy.
2023-08-01T12:26:16.225+0100 [INFO]  backend/local: apply calling Apply
2023-08-01T12:26:16.225+0100 [DEBUG] Building and walking apply graph for NormalMode plan
2023-08-01T12:26:16.226+0100 [DEBUG] ProviderTransformer: "mongodbatlas_cloud_backup_schedule.test (expand)" (*terraform.nodeExpandApplyableResource) needs provider["registry.terraform.io/mongodb/mongodbatlas"]
2023-08-01T12:26:16.226+0100 [DEBUG] ProviderTransformer: "mongodbatlas_cloud_backup_schedule.test" (*terraform.NodeApplyableResourceInstance) needs provider["registry.terraform.io/mongodb/mongodbatlas"]
2023-08-01T12:26:16.226+0100 [DEBUG] pruning unused provider["registry.terraform.io/hashicorp/aws"]
2023-08-01T12:26:16.226+0100 [DEBUG] ReferenceTransformer: "provider[\"registry.terraform.io/mongodb/mongodbatlas\"]" references: []
2023-08-01T12:26:16.226+0100 [DEBUG] ReferenceTransformer: "mongodbatlas_cloud_backup_schedule.test (expand)" references: []
2023-08-01T12:26:16.226+0100 [DEBUG] ReferenceTransformer: "mongodbatlas_cloud_backup_schedule.test" references: []
2023-08-01T12:26:16.226+0100 [DEBUG] Starting graph walk: walkApply
2023-08-01T12:26:16.226+0100 [DEBUG] created provider logger: level=debug
2023-08-01T12:26:16.261+0100 [WARN]  ValidateProviderConfig from "provider[\"registry.terraform.io/mongodb/mongodbatlas\"]" changed the config value, but that value is unused
mongodbatlas_cloud_backup_schedule.test: Modifying... [id=Y2x1c3Rlcl9uYW1l:Q2x1c3RlcjA=-cHJvamVjdF9pZA==:NjNmNGQ0YTQ3YmFlYWM1OTQwNmRjMTMx]
2023-08-01T12:26:16.269+0100 [INFO]  Starting apply for mongodbatlas_cloud_backup_schedule.test
2023-08-01T12:26:16.269+0100 [DEBUG] mongodbatlas_cloud_backup_schedule.test: applying the planned Update change
mongodbatlas_cloud_backup_schedule.test: Still modifying... [id=Y2x1c3Rlcl9uYW1l:Q2x1c3RlcjA=-cHJvamVjdF9pZA==:NjNmNGQ0YTQ3YmFlYWM1OTQwNmRjMTMx, 10s elapsed]
mongodbatlas_cloud_backup_schedule.test: Still modifying... [id=Y2x1c3Rlcl9uYW1l:Q2x1c3RlcjA=-cHJvamVjdF9pZA==:NjNmNGQ0YTQ3YmFlYWM1OTQwNmRjMTMx, 20s elapsed]
mongodbatlas_cloud_backup_schedule.test: Still modifying... [id=Y2x1c3Rlcl9uYW1l:Q2x1c3RlcjA=-cHJvamVjdF9pZA==:NjNmNGQ0YTQ3YmFlYWM1OTQwNmRjMTMx, 30s elapsed]
mongodbatlas_cloud_backup_schedule.test: Still modifying... [id=Y2x1c3Rlcl9uYW1l:Q2x1c3RlcjA=-cHJvamVjdF9pZA==:NjNmNGQ0YTQ3YmFlYWM1OTQwNmRjMTMx, 40s elapsed]
mongodbatlas_cloud_backup_schedule.test: Still modifying... [id=Y2x1c3Rlcl9uYW1l:Q2x1c3RlcjA=-cHJvamVjdF9pZA==:NjNmNGQ0YTQ3YmFlYWM1OTQwNmRjMTMx, 50s elapsed]
mongodbatlas_cloud_backup_schedule.test: Still modifying... [id=Y2x1c3Rlcl9uYW1l:Q2x1c3RlcjA=-cHJvamVjdF9pZA==:NjNmNGQ0YTQ3YmFlYWM1OTQwNmRjMTMx, 1m0s elapsed]
mongodbatlas_cloud_backup_schedule.test: Still modifying... [id=Y2x1c3Rlcl9uYW1l:Q2x1c3RlcjA=-cHJvamVjdF9pZA==:NjNmNGQ0YTQ3YmFlYWM1OTQwNmRjMTMx, 1m10s elapsed]
mongodbatlas_cloud_backup_schedule.test: Still modifying... [id=Y2x1c3Rlcl9uYW1l:Q2x1c3RlcjA=-cHJvamVjdF9pZA==:NjNmNGQ0YTQ3YmFlYWM1OTQwNmRjMTMx, 1m20s elapsed]
mongodbatlas_cloud_backup_schedule.test: Still modifying... [id=Y2x1c3Rlcl9uYW1l:Q2x1c3RlcjA=-cHJvamVjdF9pZA==:NjNmNGQ0YTQ3YmFlYWM1OTQwNmRjMTMx, 1m30s elapsed]
mongodbatlas_cloud_backup_schedule.test: Modifications complete after 1m30s [id=Y2x1c3Rlcl9uYW1l:Q2x1c3RlcjA=-cHJvamVjdF9pZA==:NjNmNGQ0YTQ3YmFlYWM1OTQwNmRjMTMx]
2023-08-01T12:27:46.300+0100 [DEBUG] State storage *statemgr.Filesystem declined to persist a state snapshot

Apply complete! Resources: 0 added, 1 changed, 0 destroyed.