[Feature flag] Enable Go implementation of UserDeleteTag

What

Enable the :go_user_delete_tag feature flag ...

Owners

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

Expectations

What release does this feature occur in first?

v13.7.0-rc2, see 8b0a877a

### What are we expecting to happen?

Same as #3370 (closed) except s/Branch/Tag/

What might happen if this goes wrong?

Same as #3370 (closed) except s/Branch/Tag/

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=UserDeleteTag&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=UserDeleteTag&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