Switch to ubuntu-24.04, since 20.04 is no longer supported

Author: hosaka

.github/workflows/main.yml

@@ -24,7 +24,7 @@ jobs:
         qt-version: ["pyside2", "pyside6", "pyqt5", "pyqt6"]
         include:
           - os: Ubuntu
-            image: ubuntu-20.04
+            image: ubuntu-24.04
           - os: Windows
             image: windows-2022
           - os: MacOS