Add REST API exposure and settings UI for MR title template (Part 3/4)

Summary

Part 3 of 4 for customizable MR title templates.

Adds the REST API exposure and project settings UI for configuring the template.

What does this MR do?

API

• Exposes mr_default_title_template on Project entity (gated by feature flag via ::Feature.enabled?)
• Gates API write-path in verify_update_project_attrs!
• Adds to entity exposure baseline and import/export allowlist
• Updates OpenAPI v2 and v3 specs

Settings UI

• Adds settings partial with beta badge in Settings > Merge Requests
• Adds partial to both CE and EE views
• Gates behind mr_default_title_template feature flag

Specs

• API request specs for feature flag on/off (GET + PUT)
• View specs for feature flag on/off

Feature flag

• Name: mr_default_title_template
• Type: beta

Related

• Depends on: Part 2 (backend logic) — merged
• Part of: #16080