In workflow, update default branch to main, use ubuntu-20.04 (#315)

* Update default branch name to `main`

* Use `ubuntu-20.04` for build, until this project upgrades to `pa11y-webservice@5`

.github/workflows/tests.yml

@@ -1,19 +1,14 @@
-# This workflow will do a clean install of node dependencies, build the source code and run tests.
-# For more information see: https://help.github.com/actions/language-and-framework-guides/using-nodejs-with-github-actions
-
 name: Build and lint
 
 on:
   push:
-    branches: # Run actions when code is committed to these branches
-      - master
+    branches:
+      - main
   pull_request:
-    branches: # Run actions when a PR is pushed based on one of these branches
-      - master
 
 jobs:
   checkout_and_test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         include: