Ensure container registry is lowercase unless starting
a sentence, bullet point, table entry, etc.

The container registry should always be lower case.

Gitaly is in the process of bumping the minimum required Git version
required by Gitaly to v2.42.0. Adapt our documentation to reflect this
change.

Since GitLab 14.0, there are not any projects using
the legacy storage, and it's sure that there won't be any
jobs of those workers in the backlog.

This MR removes all related rake tasks,
workers (first minor release), and tests that supported
the migration to the Hashed Storage in Gitlab 13.4.

Move the notes from the Omnibus doc to the respective version-specific
pages.

Fix spelling. Wrap code items in backticks.

GitLab 16.3 has a severe Geo bug, list all versions affected by this
bug. In addition, the three-level release numbering is used for reasons
of consistency.

Signed-off-by: Christian Herzig <cherzig@gauselmann.de>

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/423445

Describe a potential problem with the migration and provide options how
to fix it.

Gitaly is in the process of deprecating the `[gitlab-shell]` section in
its configuration. This section implicitly identified the location of
two different mechanisms that have been historically hosted by the
gitlab-shell project:

    - The `.gitlab_shell_secret` file, which contains the secret that is
      required to connect to the GitLab application.

    - The `custom_hooks` directory, which hosts Git hooks that shall be
      executed whenever changes are performed in a repository hosted by
      Gitaly.

These have instead been replaced by standalone items that are not
specific to the gitlab-shell project anymore. Update our documentation
to reflect the new configuration.