[Feature flag] Enable Go implementation of UserMergeBranch

What

The UserMergeBranch RPC has been ported from Ruby to Go. This issue tracks the rollout of that feature flga.

Owners

• Team: Gitaly
• Most appropriate slack channel to reach out to: #g_gitaly
• Best individual to reach out to: @pks-t

Expectations

What release does this feature occur in first?

%13.5.

### What are we expecting to happen?

No change in behaviour should occur.

What might happen if this goes wrong?

This directly impacts the merging of MRs. Failure modes:

• pipelines may not be triggered or triggered incorrectly
• generation of merge commits may fail

What can we monitor to detect problems with this?

https://dashboards.gitlab.net/d/000000199/gitaly-feature-status?orgId=1&refresh=30s&var-environment=gprd&var-method=UserMergeBranch&var-prometheus=prometheus-01-inf-gprd

Roll Out Steps

• Read the documentation of feature flags
• Enable on staging
• Test on staging
• Ensure that documentation has been updated
• Enable on GitLab.com for individual groups/projects listed above and verify behaviour
• Announce on the issue an estimated time this will be enabled on GitLab.com
• Enable on GitLab.com by running chatops command in #production
• Cross post chatops slack command to #support_gitlab-com and in your team channel
• Announce on the issue that the flag has been enabled
• Remove feature flag and add changelog entry