Update gh-pages to use ubuntu-latest

Ubuntu 18.04 is end of life today.

Author: ninjarobot

.github/workflows/gh-pages.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   deploy:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
       - name: Checkout master branch
         uses: actions/checkout@v2