partition pruning leads to different partition when `utf8mb4_general_ci ` is set

[jiyfhust]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t(col varchar(32) COLLATE utf8mb4_general_ci DEFAULT NULL) PARTITION BY KEY (`col`) PARTITIONS 7;
explain select * from t where col = 'linpin';
explain select * from t where col = 'LINPIN';

2. What did you expect to see? (Required)

The query above should be pruned to the same partition.

3. What did you see instead (Required)

mysql> explain select * from t where col = 'linpin';
+-------------------------+----------+-----------+-----------------------+--------------------------------+
| id                      | estRows  | task      | access object         | operator info                  |
+-------------------------+----------+-----------+-----------------------+--------------------------------+
| TableReader_8           | 10.00    | root      |                       | data:Selection_7               |
| └─Selection_7           | 10.00    | cop[tikv] |                       | eq(test.t.col, "linpin")       |
|   └─TableFullScan_6     | 10000.00 | cop[tikv] | table:t, partition:p4 | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+-----------------------+--------------------------------+
3 rows in set, 1 warning (0.01 sec)

mysql> explain select * from t where col = 'LINPIN';
+-------------------------+----------+-----------+-----------------------+--------------------------------+
| id                      | estRows  | task      | access object         | operator info                  |
+-------------------------+----------+-----------+-----------------------+--------------------------------+
| TableReader_8           | 10.00    | root      |                       | data:Selection_7               |
| └─Selection_7           | 10.00    | cop[tikv] |                       | eq(test.t.col, "LINPIN")       |
|   └─TableFullScan_6     | 10000.00 | cop[tikv] | table:t, partition:p3 | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+-----------------------+--------------------------------+
3 rows in set, 1 warning (0.00 sec)

partition:p4 and partition:p3, they are not same.

4. What is your TiDB version? (Required)

master

[jiyfhust]

Simalar with #51196. But not exactly same.