Upstream all golang-crypto changes and drop use of fork

I didn't see an issue for this yet, but beginning with !560 (merged) we switched golang.org/x/crypto in favor of gitlab-org/golang-crypto.

We should upstream all the changes necessary ASAP so that we can switch back to the maintained, upstream version.

As I mentioned in https://github.com/golang/crypto/pull/127#issuecomment-1247053815, we need:

• ssh: add hmac-sha2-512:
  • https://github.com/golang/crypto/pull/18 ⬇
  • https://github.com/golang/crypto/pull/257 ✅ (landed with https://github.com/golang/crypto/releases/tag/v0.11.0)
• ssh: add support for aes256-gcm@openssh.com:
  • https://github.com/golang/crypto/pull/127 ✅
• Add support for "hmac-sha2-512-etm@openssh.com":
  • https://github.com/golang/crypto/pull/129 ✅
• ssh: add support for extension negotiation (rfc 8308):
  • https://github.com/golang/crypto/pull/197 ⬇
  • https://github.com/golang/crypto/pull/211 ⬇
  • https://github.com/golang/go/issues/49269 ✅ (landing in Go 1.21)
• ssh: relax RSA signature check in SSH_MSG_USERAUTH_REQUEST:
  • https://github.com/golang/crypto/pull/221 ⬇
  • https://github.com/golang/crypto/pull/261 ⬇
  • https://go-review.googlesource.com/c/crypto/+/506835 ✅