Build docs on ubuntu-latest instead of pinned old/outdated version

Author: ksunden

.github/workflows/docs.yml

@@ -5,7 +5,7 @@ on: [push, pull_request]
 
 jobs:
   build:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
       - name: "Set up Python 3.10"