ci/linux: build AppImage on Ubuntu 22.04 (Python 3.10)

Ubuntu 20.04 only has Python 3.8, and because the 'truststore'
dependency requires at least Python 3.10, the AppImage package would
need to bundle CA certificates from 'certifi' to make HTTPS requests
work at all, which is quite an ugly workaround.

Unfortunately, as a side effect, building on Ubuntu 22.04 increases the
minimum required version of glibc from 2.31 to 2.35.

.github/workflows/ci.yml

@@ -144,7 +144,7 @@ jobs:
 
   linux-appimage:
     name: Linux (AppImage)
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
 
     steps:
       - name: Checkout code
@@ -156,12 +156,6 @@ jobs:
           echo "app_version=$(python3 -c 'from version import __version__ as v; print(v)')" >> "${GITHUB_OUTPUT}"
           echo "sha_short=$(git rev-parse --short HEAD)" >> "${GITHUB_OUTPUT}"
 
-      # Ensure Python version
-      # - name: Set up Python
-      #   uses: actions/setup-python@v4
-      #   with:
-      #     python-version: ${{env.PYTHON_VERSION}}
-
       - name: Append git revision to project version
         run: |
           sed -ri "s/^__version__\s*=\s*\"[^\"]+/\0.${{steps.vars.outputs.sha_short}}/" version.py