Bump stylesheet cache group

Author: westonruter

includes/sanitizers/class-amp-style-sanitizer.php

@@ -1645,7 +1645,7 @@ private function fetch_external_stylesheet( $url ) {
 	 */
 	private function get_parsed_stylesheet( $stylesheet, $options = [] ) {
 		$cached         = true;
-		$cache_group    = 'amp-parsed-stylesheet-v38'; // This should be bumped whenever the PHP-CSS-Parser is updated or parsed format is updated.
+		$cache_group    = 'amp-parsed-stylesheet-v39'; // This should be bumped whenever the PHP-CSS-Parser is updated or parsed format is updated.
 		$use_transients = $this->should_use_transient_caching();
 
 		// @todo If ValidationExemption::is_px_verified_for_node( $this->current_node ) then keep !important.