This improves the way similarities are plugged in in order to:
- reject the classic similarity on 7.x indices and emit a deprecation
warning otherwise
- reject unkwown parameters on 7.x indices and emit a deprecation
warning otherwise
Even though this breaks the plugin API, I'd like to backport to 7.x so
that users can get deprecation warnings when they are doing something
that will become unsupported in the future.
Closes#23208Closes#29035