[5.0.x] Fixed #34978, Refs #31331 -- Added backward incompatibility note about raw aggregations on MySQL.

Thanks Matthew Somerville for the report.

Backport of a652f07596 from main

docs/releases/4.2.txt

@@ -409,6 +409,16 @@ fields modified in the custom ``save()`` methods should be added to the
 ``update_fields`` keyword argument before calling ``super()``. See
 :ref:`overriding-model-methods` for more details.
 
+Dropped support for raw aggregations on MySQL
+---------------------------------------------
+
+MySQL 8+ allows functional dependencies on ``GROUP BY`` columns, so the
+pre-Django 4.2 workaround of grouping by primary keys of the main table is
+removed. As a consequence, using ``RawSQL()`` aggregations is no longer
+supported on MySQL as there is no way to determine if such aggregations are
+needed or valid in the ``GROUP BY`` clause. Use :ref:`aggregation-functions`
+instead.
+
 Miscellaneous
 -------------