[mob] use flutter v3.19.3

.github/workflows/mobile-release.yml

@@ -9,7 +9,7 @@ on:
             - "photos-v*"
 
 env:
-    FLUTTER_VERSION: "3.19.5"
+    FLUTTER_VERSION: "3.19.3"
 
 jobs:
     build:

mobile/README.md

@@ -46,7 +46,7 @@ You can alternatively install the build from PlayStore or F-Droid.
 
 ## 🧑‍💻 Building from source
 
-1. [Install Flutter v3.19.5](https://flutter.dev/docs/get-started/install).
+1. [Install Flutter v3.19.3](https://flutter.dev/docs/get-started/install).
 
 2. Pull in all submodules with `git submodule update --init --recursive`