update which angular version we say we support

Author: arshaw

contrib/angular/README.md

@@ -77,7 +77,7 @@ You can even supply nested templates:
 
 ## Supported Angular Versions
 
-`@fullcalendar/angular` version 6 supports Angular 12 - 18
+`@fullcalendar/angular` version 6 supports Angular 12 - 20
 
 ## Links