Fix query jumbling with GROUP BY clauses

Fix query jumbling with GROUP BY clauses

RangeTblEntry.groupexprs was marked with the node attribute
query_jumble_ignore, causing a list of GROUP BY expressions to be
ignored during the query jumbling.  For example, these two queries could
be grouped together within the same query ID:
SELECT count(*) FROM t GROUP BY a;
SELECT count(*) FROM t GROUP BY b;

However, as such queries use different GROUP BY clauses, they should be
split across multiple entries.

This fixes an oversight in 247dea89f761, that has introduced an RTE for
GROUP BY clauses.  Query IDs are documented as being stable across minor
releases, but as this is a regression new to v18 and that we are still
early in its support cycle, a backpatch is exceptionally done as this
has broken a behavior that exists since query jumbling is supported in
core, since its introduction in pg_stat_statements.

The tests of pg_stat_statements are expanded to cover this area, with
patterns involving GROUP BY and GROUPING clauses.

Author: Jian He <jian.universality@gmail.com>
Discussion: https://postgr.es/m/CACJufxEy2W+tCqC7XuJ94r3ivWsM=onKJp94kRFx3hoARjBeFQ@mail.gmail.com
Backpatch-through: 18