gitlab: Stop writing system-level gitconfig

What does this MR do?

Traditionally, multiple components used to rely on the system-level gitconfig to obtain various defaults for Git commands. Nowadays there are only two different systems that may read this configuration:

- Rugged as used in Rails. The only value that is relevant in this
  context would be `core.fsyncObjectFiles`. But because Rails
  doesn't write any Git objects into the repository this wouldn't
  have an effect anyway.

- Gitaly via either Rugged, Git2go, Git or the Ruby sidecar. Gitaly
  sets its own defaults though and makes sure to set them for all
  relevant components. Furthermore, starting with 2727686d2 (gitaly:
  Migrate to inject Git configuration via `config.toml`, 2022-06-20)
  we ask Gitaly to ignore the global gitconfig altogether in favor
  of `[[git.config]]` stanzas in its `config.toml`.

Together this means that there aren't any components left that require the global gitconfig to exist. Prune it to avoid any confusion resulting from it and add a deprecation notice.

Related to #6800 (closed).

Related to gitaly#4230 (closed).

Part of &8069 (closed).

Fixes #6889.

Related issues

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

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
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened