Proposal: Update java version and remove donwgrader

[andrej-urvantsev]

Looks like recaf is being built on Java 22 at least?

So, maybe update bento to 21 and remove downgrader?

I personally feel uneasy about using a library where byte code has been modified - introduces some kind of obscurity 🥺 .

[Col-E]

Discussed on #20 - Was originally introduced upon request for 17 support but that is no longer being asked for. I have some uncommitted work but if you want to open a PR I can rebase to that.