chore: Replace PlanOnly tests with PreApply plancheck.ExpectEmptyPlan() as advised in terraform-plugin-testing 1.6.0

[AgustinBettati]

Description

Followup up from CLOUDP-219141

Change was advised in hashicorp/terraform-plugin-testing#256.

Proposed doc changes in migration guide of terraform-plugin-testing 1.6.0, as well as migration test guide, will give the same guidance:

• website: Mention PlanOnly to ConfigPlanChecks in migration guide hashicorp/terraform-plugin-testing#261
• website: Use ConfigPlanChecks instead of PlanOnly in migration testing hashicorp/terraform-plugin-framework#899

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 checked that this change does not generate any credentials and that they are NOT accidentally logged anywhere.
• 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
• If changes include deprecations or removals, I defined an isolated PR with a relevant title as it will be used in the auto-generated changelog.

Further comments

[AgustinBettati]

leaving debug plan check as error message provided by ExpectEmptyPlan is not too helpful.

[AgustinBettati]

failing test TestAccFederatedDatabaseInstance_atlasCluster is not related (cloud-dev failure: region is currently out of capacity)

[andreaangiolillo]

LGTM