Drop explicit support of 3.26

I have no idea if things work on 3.26 so drop support, I'm also guessing that
3.26 is rarely used. 3.28 is pretty common, eg. through Ubuntu LTS.

metadata.json

@@ -4,6 +4,6 @@
   "description": "Tiling window manager with a twist",
   "url": "https://github.com/paperwm/PaperWM",
   "settings-schema": "org.gnome.Shell.Extensions.PaperWM",
-  "shell-version": [ "3.26", "3.28", "3.30"],
+  "shell-version": [ "3.28", "3.30"],
   "version": "beta"
 }