Feature flag rollout: mr_default_title_template
## Summary
Rollout issue for the `mr_default_title_template` feature flag ([beta](https://docs.gitlab.com/policy/development_stages_support/#beta)).
This feature allows project maintainers to configure a default merge request title template at the project level, supporting variables like `%{source_branch}`, `%{target_branch}`, `%{title_from_branch}`, `%{issue_title}`, `%{first_commit}`, and `%{first_multiline_commit}`.
**Feature MR:** https://gitlab.com/gitlab-org/gitlab/-/merge_requests/228442
**Feature issue:** https://gitlab.com/gitlab-org/gitlab/-/issues/16080
**Feature flag type:** `beta`
## Rollout Steps
### Enablement
- [ ] Enable on staging (`/chatops run feature set mr_default_title_template true --staging`)
- [ ] Test on staging
- [ ] Enable on GitLab.com for a percentage of projects
- [ ] Monitor error rates and performance
- [ ] Enable on GitLab.com globally (`/chatops run feature set mr_default_title_template true`)
### Cleanup
- [ ] Remove feature flag from code
- [ ] Remove feature flag YAML definition
- [ ] Remove feature flag gating from BuildService, settings view, and specs
- [ ] Add changelog entry
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