message: Fix OpenSSL 3.0 warning by switching to newer EC-curve API
  This switches the way in which the curve for a CTX is selected to a
  newer API that should work (at least) with OpenSSL 1.0, 1.1 and 3.0.
diff: @@ -231,13 +231,7 @@ SSL_CTX* session::create_ssl_context() {
  #if defined(CAF_SSL_HAS_ECDH_AUTO) && (OPENSSL_VERSION_NUMBER < 0x10100000L)
  SSL_CTX_set_ecdh_auto(ctx, 1);
  #else
  - auto ecdh = EC_KEY_new_by_curve_name(NID_secp384r1);
  - if (!ecdh)
  - CAF_RAISE_ERROR("cannot get ECDH curve");
  - CAF_PUSH_WARNINGS
  - SSL_CTX_set_tmp_ecdh(ctx, ecdh);
  - EC_KEY_free(ecdh);
  - CAF_POP_WARNINGS
  + SSL_CTX_set1_groups_list(ctx, "P-384");
  #endif
  #ifdef CAF_SSL_HAS_SECURITY_LEVEL
  const char* cipher = "AECDH-AES256-SHA@SECLEVEL=0";
  