Rollup merge of #153873 - folkertdev:deprecate-char-max, r=Mark-Simulacrum

deprecate `std::char` constants and functions

similar to how constants in those modules for numeric types have been deprecated. The `std::char` module contains:

Three stable constants that this PR deprecates. These already link to their associated constant equivalents.

- `MAX`
- `REPLACEMENT_CHARACTER`
- `UNICODE_VERSION`

two unstable constants that this PR removes. The constants are already stablized as associated constants on `char`.

- `MAX_LEN_UTF8`
- `MAX_LEN_UTF16`

Four stable functions that this PR deprecates. These already link to their method equivalents.

- `fn decode_utf16`
- `fn from_digit`
- `fn from_u32`
- `fn from_u32_unchecked⚠`

discussion at [#t-libs > should `std::char::{MIN, MAX}` be deprecated?](https://rust-lang.zulipchat.com/#narrow/channel/219381-t-libs/topic/should.20.60std.3A.3Achar.3A.3A.7BMIN.2C.20MAX.7D.60.20be.20deprecated.3F/with/579444750).

r? libs-api