Update action dependencies

actions/checkout -> v4
peaceiris/actions-mdbook -> v2

Author: arihant2math

.github/workflows/deploy.yaml

@@ -8,10 +8,10 @@ jobs:
   deploy:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - name: setup mdBook
-        uses: peaceiris/actions-mdbook@v1
+        uses: peaceiris/actions-mdbook@v2
         with:
           mdbook-version: latest