Bump GNOME runtime to 3.32

https://gitlab.gnome.org/GNOME/gnome-build-meta/tags

app.briar.gtk.json

 {
     "app-id": "app.briar.gtk",
     "runtime": "org.gnome.Platform",
-    "runtime-version": "3.28",
+    "runtime-version": "3.32",
     "sdk": "org.gnome.Sdk",
     "sdk-extensions": [
         "org.freedesktop.Sdk.Extension.openjdk10"