dev/gitaly-config: Migrate from internal socket dir to runtime dir

What does this MR do?

Gitaly has recently introduced a new runtime directory, which is where Gitaly creates all files and directories required at runtime. Migrate the development configuration to use this new setting instead of the deprecated internal socket directory.

Note that Gitaly wil create a subdirectory "gitaly-$PID" in the runtime directory. So with the previous default directory we used for the internal sockets, Gitaly now creates the directories as "/home/git/gitaly-$PID". While it might've been preferable to instead use a configuration of "/home/git/run", the configured directory must exist prior to starting. As such, it's easier to simply retain the old default for now.

Related issues

gitlab-org/gitaly#4113 (closed)

gitlab-org/gitaly!4415 (merged)

Closes gitlab-org/charts/gitlab#3205 (closed)

Checklist

See Definition of done.

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Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated