Add Groupware back-end devs as CalDAV and CardDAV code owners

This will pick us as reviews automatically

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

.github/CODEOWNERS

@@ -1,2 +1,4 @@
 */Activity/*                        @nickvergessen
 */Notifications/*                   @nickvergessen
+/apps/dav/lib/CalDAV                @ChristophWurst @miaulalala @tcitworld
+/apps/dav/lib/CardDAV               @ChristophWurst @miaulalala @tcitworld