Gitaly: update cgroups configuration

What does this MR do?

Gitaly: update cgroups configuration

In 15.0, we are updating the way that cgroups are configured. gitaly!4483 (merged) has the Gitaly changes.

This MR modifies the way the gitaly config toml generates the cgroups config according to the new format.

A second commit detects if an old version of the config is being used, and will translate it to the new config format on the Gitaly side. This way, we maintain backwards compatibility and omnibus acts as a translation layer.

NOTE: Gitaly will also remain backwards compatible with the old config as well.

Changelog: changed

Checklist

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