Apply upstream default charset change

See WordPress/WordPress@08d1cf7

> `utf8mb4` is the complete UTF-8 implementation in MySQL, supporting the full range of Unicode characters by using up to 4 bytes per character. This includes characters outside the Basic Multilingual Plane, such as emoji and supplementary characters. `utf8mb4` ensures full compatibility with modern text content and diverse character sets.
>
> As of WordPress 4.2, on both new and existing installations, WordPress would automatically upgrade database tables to `utf8mb4` on compatible servers (MySQL 5.5.3 or later). However, the default `DB_CHARSET` value during setup was still set to `utf8` for compatibility with older MySQL versions.
>
> As of WordPress 6.5, the minimum required MySQL version is 5.5.5, so the default can be updated to `utf8mb4`.

(We're currently on WordPress 6.8 here, but this default arguably makes sense to us much further back because we only support non-EOL MySQL versions.)

Author: tianon