Remove the `rsa.GenerateKey` key size restriction

[qmuntal]

We are currently falling back to Go if the RSA key size passed to rsa.GenerateKey is not 2048, 3072, nor 4096:

https://github.com/golang/go/blob/4225c6cb372e0fea7586dd646e991faa5df20671/src/crypto/rsa/rsa.go#L284

We should remove this restriction and attempt to always generate a key with the crypto backend.