[Feature flag] Enable Go implementation of UserCommitFiles

What

This issue tracks enabling go_user_commit_files feature flag and removing it after successful deployment.

Owners

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

Expectations

Operations using Gitaly's operation service's UserCommitFiles RPC continue working as expected.

What release does this feature occur in first?

%13.6

### What are we expecting to happen?

Existing usage of the RPC works.

What might happen if this goes wrong?

We might produce incorrect commits or fail to commit entirely. Failing to commit entirely should be caught by increased error rates.

What can we monitor to detect problems with this?

https://dashboards.gitlab.net/d/000000199/gitaly-feature-status?orgId=1&from=now-6h&to=now&var-environment=gprd&var-method=UserCommitFiles&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