go1.20: Replace crypto/elliptic with crypto/ecdh where possible

[aarongable]

As of go1.20, the go stdlib now includes a crypto/ecdh package which is designed to provide a safer way to use elliptic-curve cryptography.

Once we're 100% using go1.20 or above, we should go through and replace as many instances of crypto/elliptic as we can.

[aarongable]

It turns out the "where possible" portion of this bug is "nowhere".

Our two main uses for the crypto/elliptic library are:

1. Passing elliptic.P256() as an argument to ecdsa.GenerateKey(), such as here:
   

   boulder/linter/linter.go

   Line 104 in 1c785e7

   lintSigner, err = ecdsa.GenerateKey(k.Curve, rand.Reader)

   
   This functionality cannot be replicated with the new ecdh.P256().GenerateKey(), because that produces an ecdh.PrivateKey which cannot be converted to an ecdsa.PrivateKey.
2. Using elliptic.Unmarshal() to convert raw bytes into a point on a curve, such as here:
   

   boulder/pkcs11helpers/helpers.go

   Line 206 in 1c785e7

   x, y := elliptic.Unmarshal(pubKey.Curve, pointBytes)

   
   This functionality cannot be replicated with the new ecdh.Curve.NewPublicKey(), for the same reason as above.

Therefore I am closing this bug as WontFix, unfortunately.