Update Ubuntu version for CI

CI is currently failing due to:

Bikeshed now requires Python 3.9; you are on 3.8.10.
        If you're seeing this message in your CI run, you are
        likely specifying an old OS; try `ubuntu-latest`.

So try ubuntu-latest.

Author: Daniel Rubery

.github/workflows/spec-update.yml

@@ -7,7 +7,7 @@ on:
 jobs:
   main:
     name: Build, Validate and Deploy
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     permissions:
       contents: write
     steps: