Auto merge of #68325 - faern:move-numeric-consts-to-associated-consts-step1, r=LukasKalbertodt

Move numeric consts to associated consts step1

A subset of #67913. Implements the first step of RFC https://github.com/rust-lang/rfcs/pull/2700

This PR adds the new constants as unstable constants and defines the old ones in terms of the new ones. Then fix a tiny bit of code that started having naming collisions because of the new assoc consts.

Removed a test that did not seem relevant any longer. Since doing just `u8::MIN` should now indeed be valid.