upgrade from v7.5 to master, @@tidb_schema_cache_size incorrect

[tiancaiamao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Start a old version tidb cluster:

tiup playground v7.6.0 --tiflash 0

kill the tidb-server and restart a master tidb-server

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

mysql> select @@global.tidb_schema_cache_size;
+---------------------------------+
| @@global.tidb_schema_cache_size |
+---------------------------------+
| 536870912                      |
+---------------------------------+
1 row in set (0.00 sec)

mysql> select * from GLOBAL_VARIABLES where variable_name = 'tidb_schema_cache_size';
Empty set (0.01 sec)

This means the global variable @@tidb_schema_cache_size does not exist in tikv.
So the default value 536870912 is used.

Then upgrade an old tidb cluster from v7.5 to master, it will use infoschema v2.

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

[jebter]

Releted to issue: #50959