feat(postgresflex): add instance deletion protection

[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)