Remove `internal/openssh` library in favour of new `x/crypto/ssh` features

[detro]

Terraform CLI and Provider Versions

N/A / All

Use Cases or Problem Statement

As part of #151 we needed a way to marshal a Private Key into an OpenSSH-compatible PEM.

While not implemented (yet) in x/crypto/ssh, a contribution that did that was available here: we decided that, temporarily, we could keep a copy of the patch that implemented the functionality, as a local library.

Proposal

Now that x/crypto/ssh supports the marshalling into OpenSSH-compatible keys (likely because https://go-review.googlesource.com/c/crypto/+/218620 got merged), we need to cleanup and remove internal/openssh.

How much impact is this issue causing?

Low

Additional Information

Of course, the precondition to work on this, is that https://go-review.googlesource.com/c/crypto/+/218620 (or an equivalent feature) has been upstreamed.

Code of Conduct

• I agree to follow this project's Code of Conduct

[github-actions]

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