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/crystal.yml

@@ -13,7 +13,7 @@ jobs:
       - name: Install Crystal
         uses: crystal-lang/install-crystal@v1
       - name: Check out repository code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Check format
         run: crystal tool format --check
 
@@ -31,7 +31,7 @@ jobs:
         with:
           crystal: ${{ matrix.crystal }}
       - name: Check out repository code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Install dependencies
         run: shards install
       - name: Run tests

.github/workflows/documentation.yml

@@ -10,7 +10,7 @@ jobs:
     container:
       image: crystallang/crystal
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Install dependencies
         run: shards install
       - name: Generate documentation