[Feature flag] Enable Go implementation of UserDeleteBranch

What

Enable the :go_user_delete_branch feature flag ...

Owners

• Team: Gitaly
• Most appropriate slack channel to reach out to: #g_create_gitaly
• Best individual to reach out to: @avar + @chriscool

Expectations

What release does this feature occur in first?

None? It's good as of 856e2493, but that's not in a release tag yet.

### What are we expecting to happen?

For branch deletions to work as they did before, just slightly faster.

What might happen if this goes wrong?

It'll fail to delete branches, or delete the wrong branch or something. But test coverage is good, so hopefully...

What can we monitor to detect problems with this?

• staging: https://dashboards.gitlab.net/d/000000199/gitaly-feature-status?orgId=1&refresh=30s&from=now-15m&to=now&var-environment=gstg&var-method=UserDeleteBranch&var-prometheus=prometheus-01-inf-gstg
• prod: https://dashboards.gitlab.net/d/000000199/gitaly-feature-status?orgId=1&refresh=30s&from=now-15m&to=now&var-environment=gprd&var-method=UserDeleteBranch&var-prometheus=prometheus-01-inf-gprd

Beta groups/projects

If applicable, any groups/projects that are happy to have this feature turned on early. Some organizations may wish to test big changes they are interested in with a small subset of users ahead of time for example.

• gitlab-org/gitlab / gitlab-org/gitaly projects
• gitlab-org/gitlab-com groups
• ...

Roll Out Steps

• Read the documentation of feature flags
• Add featureflagstaging to this issue
• Enable on staging
• Test on staging
• Ensure that documentation has been updated
• Announce on the issue an estimated time this will be enabled on GitLab.com
• Add featureflagproduction to this issue
• 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
• Close this issue