Workflow: Use ubuntu-latest to get latest Bikeshed (#233)

Context:
speced/bikeshed#2861 (comment)

Author: inexorabletash

.github/workflows/auto-publish.yml

@@ -6,7 +6,7 @@ on:
 jobs:
   main:
     name: Build, Validate and Deploy
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
       - uses: w3c/spec-prod@v2