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-e2e.yml

@@ -10,7 +10,7 @@ on:
 
 jobs:
   cypress-matrix:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       # when one test fails, DO NOT cancel the other
       # containers, because this will kill Cypress processes
@@ -105,7 +105,7 @@ jobs:
   Cypress:
     if: ${{ always() }}
     name: Cypress (chrome)
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     needs: cypress-matrix
     steps:
       - name: Check build matrix status