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/ci.yml

@@ -44,7 +44,7 @@ jobs:
           - '1.10' # current LTS
           - '1'    # automatically expands to the latest stable 1.x release of Julia
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
       with:
         persist-credentials: false
     - uses: julia-actions/setup-julia@v2
@@ -76,7 +76,7 @@ jobs:
           - '1.10.7' # current LTS
           - '1.11.2' # currently the latest stable release
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
       with:
         persist-credentials: false
     - name: Print Docker version