unicode: update to version 15.0

[golightlyb]

This issue is intended as an umbrella tracking issue for Unicode 15 support.

Unicode 15 is now published.

Go currently supports Unicode 13 (#48621 was the issue for Unicode 14)

Additionally (#52485) CLDR support is also lagging.

[seankhliao]

cc @robpike

[mvdan]

Perhaps also cc @mpvl as he's still listed as a secondary owner in https://dev.golang.org/owners.

[robpike]

Yes, I have long forgotten the update sequence.

[smasher164]

There's a generator script in the x/text repo that has a doCore flag to update the core unicode package.
There's also a script inside x/text/unicode that only updates the rangetables.
It seems the top-level script calls all of the subpackage scripts.
Maybe after the rangetables are updated, one just copies them into the standard library?

[gopherbot]

Change https://go.dev/cl/456836 mentions this issue: all: upgrade Unicode from 13.0.0 to 15.0.0