RFR: 6594730: UUID.getVersion() is only meaningful for Leach-Salz variant

[some-java-user-99206970363698485155 at vodafonemail.de]

The following probably does not matter much because I am not an OpenJDK contributor, but personally
I think throwing an UnsupportedOperationException is reasonable:
  1. It is consistent with the other methods which also only work for one specific variant
  2. Code which calls UUID.version() on a non-variant 2 UUID is obviously already functionally broken;
     I don't think maintaining backward compatibility here adds any value

Regarding the pull request, I would recommend the following changes:
  1. Replace ` ` with a normal space, that should work as well and is easier to read
  2. Add a sentence to the method description (and not only to the `@throws` tag), that this method
     is only meaningful for variant 2 UUIDs, see for example the documentation for `timestamp()` for
     how this sentence should look like to be consistent:
     https://docs.oracle.com/en/java/javase/15/docs/api/java.base/java/util/UUID.html#timestamp()

Kind regards