Downgrade CI to ubuntu-20.04

Attempting to fix GitHub Action runner issue: The hosted runner: GitHub Actions
289 lost communication with the server. Anything in your workflow that
terminates the runner process, starves it for CPU/Memory, or blocks its network
access can cause this error.

.github/workflows/rspec.yml

@@ -16,7 +16,7 @@ on:
 jobs:
   run-rspec:
     name: Run RSpec
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Setup Postgres
         id: setup-postgres