In https://gitlab.com/gitlab-org/gitlab/-/issues/213259 we want to add
support new types of public ssh keys - `ed25519_sk`, `ecdsa_sk`.
Gems we use(https://github.com/net-ssh/net-ssh,
https://github.com/bensie/sshkey) for parsing ssh keys don't
support those types of ssh keys.

I found another gem `ssh_data` with MIT License that is well maintained.
https://github.com/github/ssh_data. This gem support all the ssh key types
we need and `ed25519_sk`, `ecdsa_sk` too.
This commit would allow us to proceed
with https://gitlab.com/gitlab-org/gitlab/-/issues/213259.

This commit removes `sshkey` gem from our dependencies.

This commit removes `SSHKeygen` from the codebase.

About `net-ssh`:
This gem remains to be used in
https://gitlab.com/gitlab-org/gitlab/-/blob/b6792b4b055cd953148dde308ae08599606efb91/app/validators/x509_certificate_credentials_validator.rb#L44.
We might be able to remove `net-ssh`, `ed25519`, `bcrypt_pbkdf`
gems from dependencies too.
See https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77424#note_812526274.
But it is better to do it in a separate commit.

Changelog: other