Remove "CHARSET" config option for MySQL, always use "utf8mb4" (#25413)

Remove "CHARSET" config option for MySQL, always use "utf8mb4" (#25413)

In modern days, there is no reason to make users set "charset" anymore.

Close #25378

## :warning: BREAKING

The key `[database].CHARSET` was removed completely as every newer
(>10years) MySQL database supports `utf8mb4` already.
There is a (deliberately) undocumented new fallback option if anyone
still needs to use it, but we don't recommend using it as it simply
causes problems.