CI: Run Flatpak jobs on release branches too

We'll soon be moving to branching before releases, which
is a case that the current Flatpak worflow did not account
for.

Adapt it to also run on release/** branches.

.github/workflows/flatpak.yml

@@ -5,13 +5,13 @@ name: Flatpak
 on:
   push:
     paths-ignore: ['**.md']
-    branches: [master]
+    branches: [master, 'release/**']
   pull_request:
     paths-ignore: ['**.md']
-    branches: [master]
+    branches: [master, 'release/**']
   release:
     types: [published]
-    branches: [master]
+    branches: [master, 'release/**']
 
 env:
   TWITCH_CLIENTID: ${{ secrets.TWITCH_CLIENT_ID }}