chore: bump actions/checkout from 4 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...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/markdownlint.yml

@@ -14,7 +14,7 @@ jobs:
         runs-on: ubuntu-latest
         steps:
             - name: Check out repository
-              uses: actions/checkout@v4
+              uses: actions/checkout@v6
 
             - name: Lint Markdown
               uses: DavidAnson/markdownlint-cli2-action@v18

.github/workflows/secret-scan.yml

@@ -14,7 +14,7 @@ jobs:
         runs-on: ubuntu-latest
         steps:
             - name: Check out repository
-              uses: actions/checkout@v4
+              uses: actions/checkout@v6
               with:
                   fetch-depth: 0
 

.github/workflows/sync-labels.yml

@@ -18,7 +18,7 @@ jobs:
         runs-on: ubuntu-latest
         steps:
             - name: Check out repository
-              uses: actions/checkout@v4
+              uses: actions/checkout@v6
 
             - name: Sync label definitions
               uses: EndBug/label-sync@v2