Upgrade actions/checkout to v6 in CI

Author: mbarbin

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Setup OCaml
         uses: ocaml/setup-ocaml@v3

.github/workflows/deploy-doc.yml

@@ -16,7 +16,7 @@ jobs:
         shell: bash
     steps:
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           fetch-depth: 0
 

.github/workflows/more-ci.yml

@@ -45,7 +45,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Setup OCaml
         uses: ocaml/setup-ocaml@v3

.github/workflows/test-deploy-doc.yml

@@ -16,7 +16,7 @@ jobs:
         shell: bash
     steps:
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           fetch-depth: 0