Changed CI image from Ubuntu 20.04 -> latest.

Changed actions/checkout from v2 -> v4.

Author: Bilge

.github/workflows/CD.yml

@@ -10,12 +10,12 @@ on:
 
 jobs:
   Build:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
         with:
           ref: gh-pages
           path: _site