upgrade from 6.1 to 6.5 will be blocked when `server-version` is set in configuration file

[lcwangchao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. setup a cluster with 2 tidb (verson 6.1) nodes with custom version using server-version in config file
2. upgrade one tidb node to 6.5
3. upgrade will be blocked

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

upgrade success

3. What did you see instead (Required)

upgrade blocked

4. What is your TiDB version? (Required)

[Benjamin2037]

After discussion, we will add a note on doc first, pingcap/docs-cn#14569, And fix this problem in later version.

[JasonWu0506]

why this bug has been fixed? if it is a doc change and decided not to fix, then we could have doc pr as fixing PR and close it properly