azurerm_dashboard_grafana - support for grafana_major_version property

[djryanj]

Added the grafana_major_version property as requested by #23666 .

Fixes #23666 .

Includes documentation update and updated acceptance tests.

Acceptance test output:

$ make acctests SERVICE='dashboard' TESTTIMEOUT='90m'
==> Checking that code complies with gofmt requirements...
==> Checking that Custom Timeouts are used...
==> Checking that acceptance test packages are used...
TF_ACC=1 go test -v ./internal/services/dashboard  -timeout 90m -ldflags="-X=github.com/hashicorp/terraform-provider-azurerm/version.ProviderVersion=acc"
=== RUN   TestAccDashboardGrafana_basic
--- PASS: TestAccDashboardGrafana_basic (583.14s)
=== RUN   TestAccDashboardGrafana_requiresImport
--- PASS: TestAccDashboardGrafana_requiresImport (655.41s)
=== RUN   TestAccDashboardGrafana_complete
--- PASS: TestAccDashboardGrafana_complete (613.18s)
=== RUN   TestAccDashboardGrafana_update
--- PASS: TestAccDashboardGrafana_update (1175.26s)
PASS
ok      github.com/hashicorp/terraform-provider-azurerm/internal/services/dashboard     3027.004s

[djryanj]

I messed up this PR; will re-open a new one.

[github-actions]

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