Renamed AsciiStr::to_lower and AsciiStr::to_upper

AsciiStr::to_lower is now AsciiStr::to_lowercase and AsciiStr::to_upper is AsciiStr::to_uppercase to match Ascii trait.

Part of issue #17790.

This is my first pull request so let me know if anything is incorrect.

Thanks!

I think the convention we've been adopting here is to make the doc comment the deprecated message.