Deprecate legacy bin/authorized_keys script

As we found out in #202 (comment 209895450), there's a legacy script that behaves the same way as gitlab-shell-authorized-keys-check but it only requires the key as parameter.

It's been replaced by gitlab-shell-authorized-keys-check in !178 (merged).

We are currently using the legacy script in our infrastructure and most likely self-managed installs are also using the old recommendation as well. We need to find a way for smooth transition for these instances.

Proposal

As mentioned in #209 (comment 312427278).

For gprd, gstg, and dev, we need to make sure that our infrastructure is using the new bin/gitlab-shell-authorized-keys-check binary.

For other users (self-managed installs), add a note to the 13.0 release post that bin/authorized_keys will be removed in the future with steps on how to use the new one.

Then we can completely remove the script in 13.1 or other succeeding milestones (this will be another issue).