Also update checkout in gh-pages.yml and opam.yml workflows

Author: jmid

.github/workflows/gh-pages.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Cache opam
         id: cache-opam

.github/workflows/opam.yml

@@ -25,7 +25,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Install OCaml compiler
         uses: ocaml/setup-ocaml@v2