chore: upgrade CI runner to Ubuntu 20.04 (#12563)

This should fix a broken CI error where apt fails to update:

  E: The repository 'https://download.opensuse.org/repositories/devel:/kubic:/libcontainers:/stable/xUbuntu_18.04  Release' no longer has a Release file.

.github/workflows/superset-python-unittest.yml

@@ -9,7 +9,7 @@ on:
 
 jobs:
   test-mysql:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python-version: [3.7]
@@ -55,7 +55,7 @@ jobs:
           bash <(curl -s https://codecov.io/bash) -cF python
 
   test-postgres:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python-version: [3.7, 3.8]
@@ -102,7 +102,7 @@ jobs:
           bash <(curl -s https://codecov.io/bash) -cF python
 
   test-sqlite:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python-version: [3.7]