ci: fix build workflow for modern GitHub Actions runners

- ubuntu-20.04 removed from GitHub Actions (all jobs stuck in queued)
  → use ubuntu-22.04
- macOS-13 → macos-latest (includes Apple Silicon)
- Python 3.9 (EOL) → 3.11
- actions/checkout v3 → v4, actions/setup-python v4 → v5
- Replace qt5-default (nonexistent on Ubuntu 22.04+) with Qt6
  runtime deps (libxcb-cursor0, libfontconfig1, libegl1)
- Add apt-get update before install

Fixes #118

Author: TimeToBuildBob

.github/workflows/build.yml

@@ -13,22 +13,22 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-20.04, windows-latest, macOS-13]
-        python_version: [3.9]
+        os: [ubuntu-22.04, windows-latest, macos-latest]
+        python_version: ['3.11']
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         submodules: 'recursive'
     - name: Set up Python
-      uses: actions/setup-python@v4
+      uses: actions/setup-python@v5
       with:
         python-version: ${{ matrix.python_version }}
     - name: Install APT dependencies
       if: runner.os == 'Linux'
       run: |
-        # libxcb-cursor0 needed since Qt 6.5
-        # https://stackoverflow.com/a/75941575/965332
-        sudo apt-get install -y qt5-default libfontconfig1 libxcb-cursor0
+        # xcb libraries needed for Qt6 on headless Linux
+        sudo apt-get update
+        sudo apt-get install -y libxcb-cursor0 libfontconfig1 libegl1
     - name: Install dependencies
       shell: bash
       run: |