Prevent gitaly process from being OOM killed due to memory hungry git

We've seen a few cases of the gitaly process getting killed when there is memory pressure on a gitaly node.

Ideally cgroup limits would always take precedence. We should investigate whether we can do more to protect the gitaly process, as killing it has a much wider impact and also has other cascading effects, such as git processes getting orphaned.

See: https://gitlab.com/gitlab-com/gl-infra/production/-/issues/8528#note_1311260400.