Follow up https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77424#note_812526274

In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77424 we started
using `ssh_data` gem instead of `net-ssh` and `sshkey`
where possible to facilitate implementation of
https://gitlab.com/gitlab-org/gitlab/-/issues/213259.

This MR eliminates the last usage of `net-ssh` from the codebase
and removes `net-ssh` and its sub dependencies - `ed25519` and `bcrypt_pbkdf`.

Fewer dependencies facilitate development and maintenance,
especially during upgrading libraries/gems.

Changelog: other