Code Quality: Replace `void` with proper return types in `wpdb` and related functions.

Replace `void` in union return types with `null`, `false`, or `never` as appropriate, and add explicit `return null` statements where methods previously fell through without a return value.

Methods updated in `wpdb`: `prepare()`, `print_error()`, `check_connection()`, `get_row()`, `get_col_info()`, `bail()`, `check_database_version()`.

Also adds `@return never` to `dead_db()` and fixes the `@phpstan-return` syntax for `wp_die()`.

Developed in https://github.com/WordPress/wordpress-develop/pull/11009

Props apermo, westonruter, xate, mukesh27, SergeyBiryukov.
Fixes #64703.


git-svn-id: https://develop.svn.wordpress.org/trunk@62177 602fd350-edb4-49c9-b593-d223f7449a82