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] <support@github.com>

Author: dependabot[bot]

.github/workflows/emacs.yml

@@ -20,7 +20,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: build lambdapi-mode
         run: make -C editors/emacs dist
       - name: install lambdapi-mode

.github/workflows/main.yml

@@ -27,7 +27,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: checking out lambdapi repo
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       # - name: recovering cached opam files
       #   uses: actions/cache@v4
       #   with:

.github/workflows/ocaml.yml

@@ -36,7 +36,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: setup-ocaml
         uses: ocaml/setup-ocaml@v3
         with:

.github/workflows/vscode.yml

@@ -27,7 +27,7 @@ jobs:
             echo "triggerPublishAndUpload=False" >> $GITHUB_OUTPUT
           fi
       - name: checking out lambdapi repo ...
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Use Node.js
         uses: actions/setup-node@v6
         with: