We have a global binding of union that returns "the union of x and y, as a list". We deprecate it because this is an outdated interface (with strong "early Python" vibes) and not suitable for the global namespace. It is only used in a few places in the Sage library. (There is no intersection.)

Depends on #32140

CC: @tscrim @jhpalmieri

Component: refactoring

Author: Matthias Koeppe

Branch/Commit: 699f05b

Reviewer: John Palmieri

Issue created by migration from https://trac.sagemath.org/ticket/32096