Add troubleshooting information for gitaly updates

Expand the merge request body with useful instructions to handle problems like red pipelines or merge conflicts.

This is a temporary actions until gitlab#227798 (closed) will be ready.

This will add the following sections to the gitaly update merge request

What should I do if this pipeline is red?

• Look at the failure, if it looks unrelated to gitaly, it's likely a master:broken or failureflaky-test.
  1. Open the pipelines tab and click Run Pipeline.
  2. This will run a pipeline for merge result
• If look gitaly related, notify the gitaly team on #g_create_gitaly slack channel.

What should I do in case of merge conflicts?

If someone updates GITALY_SERVER_VERSION with another MR, we may have a merge conflict.

In such unlikely circumstances, the better option will be closing this merge request and deleting the release-tools/update-gitaly branch.

release-tools will take care of creating a new merge request in about 1hr.