Remove qt5 from CI

Author: pifopi

.github/workflows/cpp-ci-serial-programs.yml

@@ -10,12 +10,8 @@ jobs:
       fail-fast: false
       matrix:
         os: [windows-2022, macos-13, ubuntu-24.04]
-        qt_version: ['5.12.12', '6.9.0']
+        qt_version: ['6.9.0']
         include:
-        - qt_version: '5.12.12'
-          qt_version_major: '5'
-          qt_modules: ''
-
         - qt_version: '6.9.0'
           qt_version_major: '6'
           qt_modules: 'qtmultimedia qtserialport'