Rollout eager_load_diff_collection feature flag

Summary

This is the rollout issue for the eager_load_diff_collection feature flag introduced in !227515 (merged).

The feature flag gates eager loading of DiffCollection during preload_gitaly_data, moving Gitaly gRPC streaming I/O outside the database transaction in MergeRequestMergeabilityCheckWorker.

Feature issue

https://gitlab.com/gitlab-org/gitlab/-/issues/589482

Rollout plan

• Enable on staging
• Enable on GitLab.com for a test project
• Monitor for errors and performance impact
• Enable on GitLab.com globally
• Remove feature flag