Bump actions/checkout from 5 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/ci.yaml

@@ -23,7 +23,7 @@ jobs:
 
     steps:
       - name: Checkout tree
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Set-up OCaml
         uses: ocaml/setup-ocaml@v3
@@ -40,7 +40,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout tree
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Set-up OCaml
         uses: ocaml/setup-ocaml@v3
         with:
@@ -51,7 +51,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout tree
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Set-up OCaml
         uses: ocaml/setup-ocaml@v3
         with:
@@ -62,7 +62,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout tree
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Set-up OCaml
         uses: ocaml/setup-ocaml@v3
         with: