ci(flatpak): add Flatpak runtime and base configuration for stable branch

Entire-Checkpoint: 21b9b9f90f23

electron-builder.json

@@ -72,6 +72,11 @@
     },
     "flatpak": {
         "branch": "stable",
+        "runtime": "org.freedesktop.Platform",
+        "runtimeVersion": "24.08",
+        "sdk": "org.freedesktop.Sdk",
+        "base": "org.electronjs.Electron2.BaseApp",
+        "baseVersion": "24.08",
         "files": [
             [
                 "apps/electron-backend/linux/com.fourgray.iptvnator.metainfo.xml",