Switch back to the public GitHub Actions runners

This repo was using one of the custom GitHub Actions runners when it did not need to (since it has no need of more compute, ARM CPUs or a static IP; public repos don't need a static IP for Git checkouts).

As such, it's been switched back to the standard GitHub Actions runners, as recommended here:
https://salesforce.quip.com/bu6UA0KImOxJ#temp:C:GZR57becb9df8d94f80b132168fd

.github/workflows/tests.yaml

@@ -8,7 +8,7 @@ on:
 jobs:
   tests:
     name: Tests
-    runs-on: pub-hk-ubuntu-22.04-small
+    runs-on: ubuntu-latest
 
     strategy:
       matrix: