auto-update YOURLS version in database

Addresses #2501 by checking `YOURLS_VERSION` against the stored value in the database during `yourls_upgrade_is_needed()` and triggering an update to the stored value if base code version is greater.

includes/functions.php

@@ -1060,6 +1060,10 @@ function yourls_upgrade_is_needed() {
	if( $currentsql < YOURLS_DB_VERSION )
		return true;

	// Check if YOURLS_VERSION exist && match value stored in YOURLS_DB_TABLE_OPTIONS, update DB if required
	if( $currentver < YOURLS_VERSION )
		yourls_update_option( 'version', YOURLS_VERSION );
    
	return false;
}