tidb.toml change breaks backwards compatibility

[gregwebs]

The type of a configuration parameter was change in #9625

This breaks backwards compatibility. For example, tidb-operator no longer works with the latest TiDB.
The request is to establish a technique for making type changes in the toml that are backwards compatible.
The usual approaches would be

1. Parse into multiple different types (either a string or a boolean). This is often supported for YAML, but I am not sure about doing this in TOML with go.
2. Use a new key name for the new type (I am having trouble coming up with a good name) and support the old key type

[WangXiangUSTC]

pr #9688 fix it

[morgo]

The PR referenced has been closed, so I am going to close this issue now. Please feel free to reopen it if you have any further questions. Thanks!