OpenSSL: don't set default ECDH curve

[ysbaddaden]

Disables the ECDH curve configuration that was limiting the curve selection to insecure curves.

Before (sslscan, openssl 1.1.1f, ubuntu 20.04):

  Server Key Exchange Group(s):
TLSv1.3  128 bits  secp256r1 (NIST P-256)
TLSv1.2  128 bits  secp256r1 (NIST P-256)

After (sslscan, openssl 1.1.1f, ubuntu 20.04):

  Server Key Exchange Group(s):
TLSv1.3  128 bits  secp256r1 (NIST P-256)
TLSv1.3  192 bits  secp384r1 (NIST P-384)
TLSv1.3  260 bits  secp521r1 (NIST P-521)
TLSv1.3  128 bits  x25519
TLSv1.3  224 bits  x448
TLSv1.2  128 bits  secp256r1 (NIST P-256)
TLSv1.2  192 bits  secp384r1 (NIST P-384)
TLSv1.2  260 bits  secp521r1 (NIST P-521)
TLSv1.2  128 bits  x25519
TLSv1.2  224 bits  x448

Also, ciphers now use Curve 25519 by default instead of Curve P-256

follow up to #14655
closes #9060