fix(database_cleanup): endless loop

I had a case where database_cleanup looped endlessly because of
referential integrity. DROP TABLE was failing because 2 tables had
mutual foreign keys, and the error message suggested using
DROP TABLE ... CASCADE, which indeed fixed the problem.

database_cleanup/models/purge_tables.py

@@ -72,7 +72,7 @@ def purge(self):
                     )
 
             self.logger.info("Dropping table %s", line.name)
-            self.env.cr.execute("DROP TABLE %s", (IdentifierAdapter(line.name),))
+            self.env.cr.execute("DROP TABLE %s CASCADE", (IdentifierAdapter(line.name),))
             line.write({"purged": True})
         return True