jekyll.yml: try ubuntu 20

Author: ilg-ul

.github/workflows/jekyll.yml

@@ -9,7 +9,7 @@ on:
 jobs:
   build:
 
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
 
     env:
       GITHUB_DEST_REPO: "micro-os-plus/micro-os-plus.github.io"