Update PostgreSQL version requirements documentation for 19.0
## Summary
Update documentation to reflect that PostgreSQL 17 is the minimum required version in GitLab 19.0, and PostgreSQL 16 is no longer supported.
Related: #9725
## Precedent: PG14/15 removal in 18.0
- gitlab!190814 — bumped minimum PG version to 16 in `lib/gitlab/database.rb` and updated the version matrix in `doc/install/requirements.md`
## Scope
- Update `doc/install/requirements.md` version matrix to show:
- 19.x: minimum PostgreSQL 17.x
- 18.x: minimum PostgreSQL 16.x (no change, but verify max is set to 17.x)
- Update `lib/gitlab/database.rb` `MINIMUM_POSTGRES_VERSION` from 16 to 17
- Update any related specs (`spec/lib/gitlab/database/reflection_spec.rb`)
- Verify `doc/update/deprecations.md` is accurate (generated via rake task)
## Acceptance Criteria
- [ ] `MINIMUM_POSTGRES_VERSION` set to 17
- [ ] Requirements documentation reflects PG17 as minimum for 19.x
- [ ] Related specs updated and passing
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