support Unicode 15.1 via utf8proc 2.9.0 (#51799)
Similar to #47392, support [Unicode
15.1](https://www.unicode.org/versions/Unicode15.1.0/) by bumping
utf8proc to 2.9.0 (https://github.com/JuliaStrings/utf8proc/issues/253).
This allows us to use [118 exciting new emoji
characters](https://blog.emojipedia.org/whats-new-in-unicode-15-1-and-emoji-15-1/)
as identifiers, including "edible mushroom" `"\U1f344\u200d\U1f7eb"`
(but still no superscript "q").
Interestingly, they also updated the [Unicode recommendations on
programming-language identifiers
(UAX#31)](https://www.unicode.org/reports/tr31/tr31-39.html#Mathematical_Compatibility_Notation_Profile)
to finally "bless" identifiers beginning with `∂` and `∇` and/or ending
with numeric sub/superscripts. They still don't recommend nearly the
range of identifiers accepted by Julia, however.
