Meta: update build to use ubuntu-latest

Author: deniak

.github/workflows/build-validate-publish.yml

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