chore(framework/pylons): pin github actions ubuntu runner to 20.04 (#4619) (#4622)

(cherry picked from commit fdcefa9)

Co-authored-by: Yun Kim <[email protected]>

Author: mergify[bot]

.github/workflows/test_frameworks.yml

@@ -380,7 +380,8 @@ jobs:
 
   pylons-testsuite-1_0_3:
     name: Pylons 1.0.3
-    runs-on: "ubuntu-latest"
+    runs-on: "ubuntu-20.04"
+    # Ubuntu 20.04 is the last version of ubuntu on github setup actions to provide Python 2.7.
     env:
       DD_TESTING_RAISE: true
       PYTHONPATH: ../ddtrace/tests/debugging/exploration/