ci: 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

@@ -38,7 +38,7 @@ jobs:
           - x64
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - uses: julia-actions/setup-julia@v2
         with:
@@ -226,7 +226,7 @@ jobs:
     timeout-minutes: 45
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - uses: julia-actions/setup-julia@v2
         with:
@@ -318,7 +318,7 @@ jobs:
       statuses: write
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - uses: julia-actions/setup-julia@v2
         with:

.github/workflows/mpi-examples.yml

@@ -17,7 +17,7 @@ jobs:
         version: ['1']
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - uses: julia-actions/setup-julia@v2
         with: