With the major Shadow 9.x update, the fat JMH JAR wasn't correctly created. Found that out when our JMH pipeline failed, our runner couldn't find the main class to start the tests.

Shadow 9.x has many breaking changes, I suspect some of them changed default custom task config behaviour. This PR explicitly sets JMH configuration instead.

Note that you can't "test" this with our unit tests, they are passing but doesn't indicate anything. The real test is if we can run our JMH performance pipeline off this pull request.

I will monitor our perf pipeline after this gets merged in