build(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] <support@github.com>

Author: dependabot[bot]

.github/workflows/CI.yml

@@ -38,7 +38,7 @@ jobs:
         arch:
           - x64
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: julia-actions/setup-julia@v2
         with:
           include-all-prereleases: true
@@ -62,7 +62,7 @@ jobs:
       contents: write
       statuses: write
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: julia-actions/setup-julia@v2
         with:
           version: '1.11'

.github/workflows/IntegrationTest.yml

@@ -25,7 +25,7 @@ jobs:
                     - { user: ReactiveBayes, repo: RxInfer.jl }
 
         steps:
-            - uses: actions/checkout@v5
+            - uses: actions/checkout@v6
 
             - uses: julia-actions/setup-julia@v2
               with:
@@ -35,7 +35,7 @@ jobs:
             - uses: julia-actions/julia-buildpkg@v1
 
             - name: Clone Downstream
-              uses: actions/checkout@v5
+              uses: actions/checkout@v6
               with:
                   repository: ${{ matrix.package.user }}/${{ matrix.package.repo }}
                   path: downstream