Auto merge of #130738 - bjoernager:const-make-ascii, r=jhpratt

Mark `make_ascii_uppercase` and `make_ascii_lowercase` in `[u8]` and `str` as const.

Relevant tracking issue: #130698

This PR extends #130697 and #130713 to the similar methods in byte slices (`[u8]`) and string slices (`str`).

For the `str` methods, this simply requires adding the `const` specifier to the function signatures. The `[u8]` methods, however, require (at least a temporary) reimplementation due to the use of iterators and `for` loops.

Author: bors