Use processed group clause in PG16 (take 2)

[fabriziomello]

In #6377 we fixed an ORDER BY/GROUP BY expression not found in targetlist by using the root->processed_groupClause instead of parse->groupClause due to an optimization introduced in PG16 that removes redundant grouping and distinct columns.

But looks like we didn't change all necessary places, specially our HashAggregate optimization.

Disable-check: force-changelog-file

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 81.76%. Comparing base (59f50f2) to head (bcf05ca). Report is 237 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #7091 +/- ## ========================================== + Coverage 80.06% 81.76% +1.69% ========================================== Files 190 200 +10 Lines 37181 37323 +142 Branches 9450 9738 +288 ========================================== + Hits 29770 30516 +746 + Misses 2997 2894 -103 + Partials 4414 3913 -501 ```

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