Stabilize assoc_int_consts associated int/float constants
The next step in RFC https://github.com/rust-lang/rfcs/pull/2700 (tracking issue #68490). Stabilizing the associated constants that were added in #68325.
* Stabilize all constants under the `assoc_int_consts` feature flag.
* Update documentation on old constants to say they are soft-deprecated and the new ones should be preferred.
* Update documentation examples to use new constants.
* Remove `uint_macro` and use `int_macro` for all integer types since the macros were identical anyway.
r? @LukasKalbertodt