In modern days, there is no reason to make users set "charset" anymore.
Close#25378
## ⚠️ 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.
