Remove the role validation for Postgres

[Agraphie]

Description

This allows the validation to happen on the API side. Currently, the API already allows creation with the "createrole", but the terraform provider doesn't. This is in-line with the change done in fda9622 for sql-server.

relates to #1105

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[rubenhoenle]

@Agraphie Thanks for the contribution!

Could you please check the failing CI pipeline? There seems to be some linter issue.

[Agraphie]

@rubenhoenle thanks for checking. I forgot to run the linting locally, my bad. Linter approved now locally.