deps: Update to Qt 6.6.0 and SDL2 2.28.4

Also bumps the minimum macOS version to 11.0.

Minimum for building is 6.5.3, because there's no 6.6 flatpak runtime yet.

.github/workflows/scripts/linux/flatpak/modules/20-sdl2.json

@@ -28,8 +28,8 @@
   "sources": [
     {
       "type": "archive",
-      "url": "https://libsdl.org/release/SDL2-2.28.2.tar.gz",
-      "sha256": "64b1102fa22093515b02ef33dd8739dee1ba57e9dbba6a092942b8bbed1a1c5e"
+      "url": "https://libsdl.org/release/SDL2-2.28.4.tar.gz",
+      "sha256": "888b8c39f36ae2035d023d1b14ab0191eb1d26403c3cf4d4d5ede30e66a4942c"
     }
   ],
   "cleanup": [