wallpapers/color: Add back "ServiceTypes" property in metadata.json

In containmentconfigview.cpp, "Plasma/Wallpaper" is checked if a plugin
is of ServiceType Plasma/Wallpaper.

BUG: 448625
FIXED-IN: 5.24


(cherry picked from commit 5c8816ddc7)

wallpapers/color/metadata.json

@@ -113,6 +113,9 @@
         "Name[x-test]": "xxPlain Colorxx",
         "Name[zh_CN]": "纯色",
         "Name[zh_TW]": "普通顏色",
+        "ServiceTypes": [
+            "Plasma/Wallpaper"
+        ],
         "Website": "https://kde.org/plasma-desktop"
     },
     "Keywords": "",