Development Task

In previous work #17311 and relevant doc https://github.com/pingcap/docs-cn/pull/18336/files, the default "region-size" has been enlarged to 256MiB from 96MiB, for a better performance.

However, this change does not consider that if the user does not set the relevant setting manually, the default region-size will be changed to the latest one -- 256 MiB, whatever the previous setting is. That is, this change does not take the inheritance of "region-size" configurations into consideration, which is not an elegant change as expected.

It's better to introduce the enhancement that the region-size should inherit from the last config file if necessary.