data: Switch flatpak build to using a defined platform version

At some point we were going to regret just specifying master.

Author: kaiw

data/org.gnome.meld-devel.json

@@ -1,7 +1,7 @@
 {
   "app-id": "org.gnome.meld",
   "runtime": "org.gnome.Platform",
-  "runtime-version": "master",
+  "runtime-version": "3.28",
   "sdk": "org.gnome.Sdk",
   "command": "meld",
   "cleanup": [