community/polymc: move from testing, upgrade to 1.4.1, use Qt6
The package has been in testing for about 6 months now, and I haven't found any issues while using it.
(cc @PureTryOut - let me know if you're fine with polymc being moved to community, I can revert that part of the MR if needed. (I'd also be willing to take over maintainership if it's easier for you :p))