Auto merge of #130697 - bjoernager:const-char-make-ascii, r=dtolnay

Mark `char::make_ascii_uppercase` and `char::make_ascii_lowercase` as const.

Relevant tracking issue: #130698

The `make_ascii_uppercase` and `make_ascii_lowercase` methods in `char` should be marked "const."

With the stabilisation of [`const_mut_refs`](rust-lang/rust#57349), this simply requires adding the `const` specifier to the function signatures.

Author: bors