UUIDv6 instead of time-ordered v4

[tillkruss]

It would be nice to use the official UUID Version 6 instead of the time-ordered v4 in an upcoming major release.

Both are sortable, but v6 makes decoding the timestamp much easier.

• Add Str::uuid6() helper
• Use v6 in in HasUuids::newUniqueId()

[juse-less]

UUIDv7 was merged for Laravel 10.
It's essentially the same as UUIDv6, but uses unix timestamps for the timestamp part.

#44210