Update deprecation with docs to upgrade from legacy Gitaly config

What does this MR do and why?

Related to #393574 (closed)

This MR updates the deprecation notice introduced by Deprecate gitaly legacy config (!113286 - merged) with documentation specific to how to upgrade the Gitaly configuration to meet the new format.

This MR is created as a follow on because we wanted to announce the deprecation as soon as possible, but we don't want to forget to add the appropriate upgrade steps (hence the creation of this follow-on MR).

Technical writer merge process

The deprecations doc's .md file must be updated before this MR is merged:

1. Check out the MR's branch (in the gitlab-org/gitlab project).
2. From the command line (in the branch), run bin/rake gitlab:docs:compile_deprecations. If you want to double check that it worked, you can run bin/rake gitlab:docs:check_deprecations to verify that the doc is up to date.
3. Commit the updated file and push the changes.
4. Set the MR to merge when the pipeline succeeds (or merge if the pipeline is already complete).

If you have trouble running the Rake task, check the troubleshooting steps.