ci: use ubuntu-latest for Python matrix

Lowest Python version is 3.9, and is available on ubuntu-latest

Link: https://launchpad.net/ubuntu/oracular/+source/python-aiohttp/+builds
Signed-off-by: Ariel Otilibili <otilibil@eurecom.fr>

Author: ariel-anieli

.github/workflows/ci.yml

@@ -36,7 +36,7 @@ jobs:
       - run: make check-commits
 
   test:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     strategy:
       matrix:
         include: