Database: Update default DB_CHARSET to utf8mb4 in wp-config-sample.php.

`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`.

Follow-up to [30345], [31349], [57173], [57926].

Props bchecketts, JavierCasares, dmsnell, SergeyBiryukov.
Fixes #48285.

git-svn-id: https://develop.svn.wordpress.org/trunk@60629 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

wp-config-sample.php

@@ -32,7 +32,7 @@
 define( 'DB_HOST', 'localhost' );
 
 /** Database charset to use in creating database tables. */
-define( 'DB_CHARSET', 'utf8' );
+define( 'DB_CHARSET', 'utf8mb4' );
 
 /** The database collate type. Don't change this if in doubt. */
 define( 'DB_COLLATE', '' );

wp-tests-config-sample.php

@@ -41,7 +41,7 @@
 define( 'DB_USER', 'yourusernamehere' );
 define( 'DB_PASSWORD', 'yourpasswordhere' );
 define( 'DB_HOST', 'localhost' );
-define( 'DB_CHARSET', 'utf8' );
+define( 'DB_CHARSET', 'utf8mb4' );
 define( 'DB_COLLATE', '' );
 
 /**#@+