deprecate: optional root project_id attribute in mongdbatlas_project_api_key is deprecated

[AgustinBettati]

Description

As a follow up to #1664, this PR marks the adjusted attribute as deprecated. Created in separate PR to have explicit entry in change log.

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

[github-actions]

Package	Line Rate	Health
github.com/mongodb/terraform-provider-mongodbatlas/internal/common/conversion	31%	❌
github.com/mongodb/terraform-provider-mongodbatlas/internal/common/validate	68%	➖
github.com/mongodb/terraform-provider-mongodbatlas/internal/provider	5%	❌
github.com/mongodb/terraform-provider-mongodbatlas/internal/service/advancedcluster	3%	❌
github.com/mongodb/terraform-provider-mongodbatlas/internal/service/alertconfiguration	0%	❌
github.com/mongodb/terraform-provider-mongodbatlas/internal/service/atlasuser	0%	❌
github.com/mongodb/terraform-provider-mongodbatlas/internal/service/cluster	1%	❌
github.com/mongodb/terraform-provider-mongodbatlas/internal/service/databaseuser	0%	❌
github.com/mongodb/terraform-provider-mongodbatlas/internal/service/encryptionatrest	0%	❌
github.com/mongodb/terraform-provider-mongodbatlas/internal/service/project	0%	❌
github.com/mongodb/terraform-provider-mongodbatlas/internal/service/projectipaccesslist	0%	❌
github.com/mongodb/terraform-provider-mongodbatlas/internal/service/searchdeployment	23%	❌
github.com/mongodb/terraform-provider-mongodbatlas/internal/testutil/acc	6%	❌
github.com/mongodb/terraform-provider-mongodbatlas/mongodbatlas	2%	❌
Summary	3% (315 / 10692)	❌

[AgustinBettati]

Verified failing test are not related (alert config and database user)