Make "Compatibility level" log to debug.

[modmuss50]

Finally annoyed me enough to make a PR, I dont think it provides meaningful enough info to always be logged.

Let me know if you disagree 👍

[liach]

I recall Mumfrey mentioned long ago that this CompatibilityLevel thing is quite useless. It should simply be taken care as part of the mod's java version requirements. And as I see, CompatibilityLevel and LanguageFeatures are quite in disarray and most likely isn't working as intended (outdated versions, incorrect max supported, incomplete language feature scan), I would recommend deprecating them altogether for an eventual removal (assuming mixin will still be alive by then)