pkey/ec: deprecate PKey::EC::Point#make_affine! and make it a no-op

It converts the internal representation of the point object to the
affine coordinate system. However, it had no real use case because the
difference in the internal representation has not been visible from
Ruby/OpenSSL at all.

EC_POINT_make_affine() is marked as deprecated in OpenSSL 3.0.

ext/openssl/ossl_pkey_ec.c

@@ -1252,6 +1252,8 @@ static VALUE ossl_ec_point_is_on_curve(VALUE self)
 /*
  * call-seq:
  *   point.make_affine! => self
+ *
+ * This method is deprecated and should not be used. This is a no-op.
  */
 static VALUE ossl_ec_point_make_affine(VALUE self)
 {
@@ -1261,8 +1263,11 @@ static VALUE ossl_ec_point_make_affine(VALUE self)
     GetECPoint(self, point);
     GetECPointGroup(self, group);
 
+    rb_warn("OpenSSL::PKey::EC::Point#make_affine! is deprecated");
+#if !OSSL_OPENSSL_PREREQ(3, 0, 0)
     if (EC_POINT_make_affine(group, point, ossl_bn_ctx) != 1)
         ossl_raise(cEC_POINT, "EC_POINT_make_affine");
+#endif
 
     return self;
 }