ci: Add Ubuntu 26.04 job to cover Qt >= 6.5 code path

The existing Ubuntu 24.04 job builds against Qt 6.4, which exercises
only the `Qt.labs.settings` variant of the `AppSettings` type. Add a
job with Qt 6.10 to also cover the `QtCore` variant.

Author: hebasto

.github/workflows/ci.yml

@@ -8,7 +8,17 @@ on:
 
 jobs:
   linux:
-    runs-on: ubuntu-24.04
+    name: ${{ matrix.job_name }}
+    runs-on: ${{ matrix.image }}
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - job_name: 'Ubuntu 24.04, Qt 6.4'
+            image: ubuntu-24.04
+          - job_name: 'Ubuntu 26.04, Qt 6.10'
+            image: ubuntu-26.04
+
     env:
       BUILD_APP_TESTS: ON
     steps: