Remove populators for Application, Client, Gateway, Organization and User

[htdvisser]

Summary

This removes the populator options for entities used in the Identity Server.

While working on the test for public fields, I also thought I'd clarify which fields are public in the documentation, so I updated the field comments.

Refs #342

Changes

• Stop using populators in IS
• Stop using populators in public field test
• Improve documentation of public fields
• Remove Application, Client, Gateway, Organization and User populators

Testing

Existing tests

Regressions

None expected; populators were never used in production code.

Notes for Reviewers

Scheduled this for v3.14.2 but depending on time, it may still land in v3.14.1.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.