Declare compatibility with GNOME 47

According to https://gjs.guide/extensions/upgrading/gnome-shell-47.html
there are no major changes, so this extension should just work.

Let's see.

metadata.json

@@ -4,7 +4,7 @@
   "description": "Get a picture of the day as desktop background or wallpaper.\n\nSupports the following sources: NASA Astronomy Picture of the Day (APOD), NASA Earth Observatory Image of the Day, Bing, Wikimedia Featured Image, and Simon Stålenhag artworks.",
   "url": "https://github.com/swsnr/gnome-shell-extension-picture-of-the-day",
   "version-name": "46.6",
-  "shell-version": ["46"],
+  "shell-version": ["46", "47"],
   "settings-schema": "org.gnome.shell.extensions.swsnr-picture-of-the-day",
   "gettext-domain": "[email protected]"
 }