Better highlighting of archved labels (#29749)

as followup of the not jet finished discussion at
https://github.com/go-gitea/gitea/pull/29680#discussion_r1521867261

we enhance and chat about how best to highlight archived labels here :)

---------

Co-authored-by: silverwind <me@silverwind.io>

web_src/css/repo.css

@@ -2410,7 +2410,8 @@
 }
 
 .archived-label {
-  filter: grayscale(0.8);
+  filter: grayscale(0.5);
+  opacity: 0.5;
 }
 
 .ui.label.scope-left {