Enable mr_ai_resolve_conflicts by default

What does this MR do?

Transitions the mr_ai_resolve_conflicts feature flag from wip to beta and sets default_enabled: true, enabling AI-assisted merge conflict resolution for all users by default.

The flag remains in place so administrators can disable it if needed.

Why

The feature has completed its initial WIP phase and is ready for broad rollout. Per the feature flag lifecycle, wip flags must transition to beta (or gitlab_com_derisk) before being defaulted to true.

Changes

• Moved config/feature_flags/wip/mr_ai_resolve_conflicts.yml to config/feature_flags/beta/
• Changed type: wip to type: beta
• Changed default_enabled: false to default_enabled: true

References

• Rollout issue: #596465
• Feature issue: #588530 (closed)
• Introduced in: !230512 (merged)