build: CI and the snap build with Flutter v3.47.8, as the laptop does

A patch release; the release tool refuses to build with another Flutter
than CI tests with.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

.forgejo/workflows/ci.yaml

@@ -23,7 +23,7 @@ jobs:
     timeout-minutes: 45
     env:
       # The Flutter the app is developed with; move it together with the local one.
-      FLUTTER_VERSION: 3.47.5
+      FLUTTER_VERSION: 3.47.8
       # The Serverpod CLI, for the generated-code check: the servers' Serverpod version
       # (every package pins it; the root's tests hold it to the lock); move it together
       # with them.