Refactor to use function in `wpdb` to find `charset_collate`

[sudar]

The wp_get_db_schema function In wp-admin/schema.php is using the same code from wpdb->get_charset_collate() to find out the charset_collate.

This patch replaces the duplicate code with a call to wpdb->get_charset_collate().

This doesn't make any change to the functionality but cleans up the code, so that in future if the logic to find charset_collate is changed, then it needs to be changed in only one place.

[pento]

In 37524:

Database: Remove some duplicate code.

schema.php was manually defining the character set/collation query, instead of using wpdb::get_charset_collate().

Props sudar.

Fixes #35756.