[MarcAd1234]

Description

Adds deletion_protection to stackit_postgresflex_instance (optional) and the data source .

• The create API doesn't accept the flag, so protection is enabled via POST .../protections once the instance is ready.
• terraform destroy fails early with a clear message if the instance is protected.
• If the attribute isn't set, the current value from the API is kept, so existing configs show no diff after upgrading

relates to #1800

FYI: TestAccPostgresFlexInstanceMax passes create, data source and import, but the update step fails on 5 pre-existing checks so this seems to be an issue in the test case rather than in this change.

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
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[github-actions]

This PR was marked as stale after 14 days of inactivity and will be closed after another 14 days of further inactivity. If this PR should be kept open, just add a comment, remove the stale label or push new commits to it.

[HartmannVolker]

What is blocking this PR?

[MarcAd1234]

Rebased onto current main;
still waiting for review