Remove `net-ssh`, `ed25519`, and `bcrypt_pbkdf` gems from dependencies

What does this MR do and why?

Follow up !77424 (comment 812526274)

In !77424 (merged) we started using ssh_data gem instead of net-ssh and sshkey where possible to facilitate implementation of #213259 (closed).

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

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How to set up and validate locally

bin/rspec spec/validators/x509_certificate_credentials_validator_spec.rb

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