planner: update the default value of tidb_ignore_inlist_plan_digest to True (#22516)

Author: qw4990

system-variables.md

@@ -3523,11 +3523,11 @@ For a system upgraded to v5.0 from an earlier version, if you have not modified
 - Persists to cluster: Yes
 - Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
 - Type: Boolean
-- Default value: `OFF`
+- Default value: `ON`. Before v8.5.6, the default value is `OFF`.
 - This variable controls whether TiDB ignores the element differences in the `IN` list across different queries when generating Plan Digests.
 
-    - When it is the default value `OFF`, TiDB does not ignore the element differences (including the difference in the number of elements) in the `IN` list when generating Plan Digests. The element differences in the `IN` list result in different Plan Digests.
-    - When it is set to `ON`, TiDB ignores the element differences (including the difference in the number of elements) in the `IN` list and uses `...` to replace elements in the `IN` list in Plan Digests. In this case, TiDB generates the same Plan Digests for `IN` queries of the same type.
+    - When it is the default value `ON`, TiDB ignores the element differences (including the difference in the number of elements) in the `IN` list and uses `...` to replace elements in the `IN` list in Plan Digests. In this case, TiDB generates the same Plan Digests for `IN` queries of the same type.
+    - When it is set to `OFF`, TiDB does not ignore the element differences (including the difference in the number of elements) in the `IN` list when generating Plan Digests. The element differences in the `IN` list result in different Plan Digests.
 
 ### tidb_index_join_batch_size