Deprecate git_data_dirs.<storage>.path

The following discussion from !6808 (merged) should be addressed:

• @samihiltunen started a discussion: (+3 comments)

  @balasankarc The fix makes sense, don't set the old key and rely on the conversion code but directly set the new key instead.

  That said, is it necessary to set the git_data_dirs.<storage>.path to begin with? The flow is somewhat complicated:

  1. Set the default storage in git_data_dirs.
  2. Configure Rails with the storage.
  3. If Gitaly storages are unconfigured, configure them from git_data_dirs.

  Ideally it would be:

  1. Set a default for gitaly[:configuration][:storage]
  2. Rails doesn't need the path.
     • Ideally the path shouldn't need to be configured, just the address, but I guess Rails still needs it for Rugged patches?

  I think we should post 16.0 deprecate git_data_dirs[<storage>].path to simplify this. This way we'd only have one way to configure Gitaly storages, not two.

  Looks good to me, nothing blocking but we should remove git_data_dirs.path in the future.