Correctly mark config in spanner instance as ForceNew

[slevenick]

Fixes: hashicorp/terraform-provider-google#7732

This is not updatable and the API throws an error if you try.

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• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

spanner: marked `google_spanner_instance.config` as ForceNew as is not updatable

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 1 file changed, 1 insertion(+), 6 deletions(-))
Terraform Beta: Diff ( 1 file changed, 1 insertion(+), 6 deletions(-))
Ansible: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=156817"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccDataSourceSpannerInstance_basic|TestAccApiGatewayApiConfigIamBindingGenerated|TestAccApiGatewayApiConfigIamMemberGenerated|TestAccApiGatewayApiConfigIamPolicyGenerated|TestAccApiGatewayGatewayIamBindingGenerated|TestAccApiGatewayGatewayIamPolicyGenerated|TestAccApiGatewayGatewayIamMemberGenerated|TestAccApiGatewayApiConfig_apigatewayApiConfigBasicExample|TestAccApiGatewayApiConfig_apigatewayApiConfigBasicExampleUpdated|TestAccApiGatewayGateway_apigatewayGatewayBasicExample|TestAccApiGatewayGateway_apigatewayGatewayBasicExampleUpdated|TestAccActiveDirectoryDomainTrust_activeDirectoryDomainTrustBasicExample|TestAccBigqueryDataTransferConfig|TestAccCloudRunService_cloudRunServiceUpdate|TestAccComposerEnvironment_withUpdateOnCreate|TestAccFilestoreInstance_filestoreInstanceBasicExample|TestAccFilestoreInstance_update|TestAccOSLoginSSHPublicKey_osLoginSshKeyExpiry You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=156831"