Updating a user should invalidate userslugs cache

[thebrandonallen]

There are four user cache groups. However, between wp_insert_user and wp_update_user, only three of the groups are invalidated. The userslugs cache is never invalidated. Since a nicename can be updated via wp_update_user we should be invalidating the userslugs cache as well.

[boonebgorges]

Good catch.

[boonebgorges]

In 36482:

When updating a user, invalidate its 'userslugs' cache.

user_nicename can be changed via wp_update_user(), so we invalidate just
to be safe.

Props thebrandonallen.
Fixes #35750.