[gh-actions](deps): 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/minimum-vcs.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Check out repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Build Docker image
         run: docker build -t minimum-vcs .github/workflows/tools/minimum-vcs

.github/workflows/no-vcs.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Check out repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Build Docker image
         run: docker build -t no-vcs .github/workflows/tools/no-vcs

.github/workflows/test.yml

@@ -47,7 +47,7 @@ jobs:
             os: ubuntu-latest
     steps:
       - name: Check out repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Install Mercurial
         # Mercurial seems to be broken on PyPy, so we need to install it before