ci: Set runner version to Ubuntu 22.04 LTS

Addresses recent change of 'ubuntu-latest' runner
bumped to Ubuntu 24.04 LTS. More details here:
actions/runner-images#10636

This fixes Python 3.7 issue in linting action.

Signed-off-by: Alex Gavin <a_gavin@icloud.com>

.github/workflows/ci.yml

@@ -5,7 +5,9 @@ on: [push, pull_request]
 # Lint code using flake8 and run unit tests using pytest
 jobs:
   lint:
-    runs-on: ubuntu-latest
+    # Tie runner to Ubuntu 22.04 LTS, as it still supports Python 3.7
+    # Minimum Python version on Ubuntu 24.04 LTS is Python 3.9.
+    runs-on: ubuntu-22.04
     name: Run Linter
     steps:
       - name: Check out source repository
@@ -17,4 +19,4 @@ jobs:
       - name: flake8 Lint
         uses: py-actions/flake8@v2
         #with:
-        #  plugins: "flake8-bugbear"
+        #  plugins: "flake8-bugbear"