Remove the #[allow(non_uppercase_statics)] attribute from inside bitflags!

Closes #17773.
It's a little unfortunate that this has to be applied individually to each static...

I also think that to follow our own naming conventions we'd want to rename these to UPPER_SNAKE_CASE anyway. cc @aturon, we haven't touched I/O in terms of stability yet, do you think it would be premature to start renaming these?