deprecate the color_u8! macro and make methods const

[cyrgani]

With Rust 1.82, const_fn_floating_point_arithmetic is stable (see rust-lang/rust#57241).

This means that Color::from_rgba can be const and the color_u8! macro is not required anymore. Color::from_rgba_f32 was added too since the macro allowed passing f32 as well as u8 (or anything else, actually).

Note that this should only be merged after the release of 1.82.