Bump actions/checkout from 3 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v3...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/main.yml

@@ -8,7 +8,7 @@ jobs:
   test:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v6
       with:
         fetch-depth: 0
     - name: Running Validate Content

.github/workflows/s3sync.yml

@@ -14,7 +14,7 @@ jobs:
     name: Sync simpleitk bucket
     runs-on: self-hosted
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v6
     - name: Install dependencies
       run: |
         sudo apt-get update -y