Changelog: deprecated

We recently deprecated the `omnibus_gitconfig` configuration key in
GitLab 16.10. The initial version of the change was referring to the
key by using the internal name in Omnibus rather than the key the end
user would be configuring. The body of the deprecation item was updated
but the old key was left in the title. Fix the title to refer to the
correct key as well.

This commit adds a deprecation notice and migration instructions
for `omnibus_gitconfig`.

Lots of links that needed fixing in these older YAML files, which is
not exactly surprising.

The link text lacked a destination. I changed the linking text to
match the subheading title for the endpoint, and used the link.

This reverts merge request !145480

https://gitlab.com/gitlab-org/gitlab/-/issues/388424

Run the rake task, update the page, get a green pipeline!