Reflect the actual implementation from gitlab!237869 and
gitlab!237907:

- Storage: settings live in a `logging_settings` JSONB column using
  `jsonb_accessor`, not separate columns
- Feature flag: gated behind `logging_field_variant_versioning` (WIP)
- Validation rules: no downgrade, dual-emit > schema version, no zero
- API: exposed via PUT /api/v4/application/settings
- Updated ServiceMapping status and added implementation references

docs(fields): Add logging field variant versioning documentation

See merge request !514

Merged-by: Matias Alvarez <malvarez@gitlab.com>
Approved-by: Luke Hollinda <lhollinda@gitlab.com>
Reviewed-by: GitLab Duo <gitlab-duo@gitlab.com>
Reviewed-by: Matias Alvarez <malvarez@gitlab.com>