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] <[email protected]>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -34,7 +34,7 @@ jobs:
     steps:
       -
         name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       -
         name: Docker meta
         id: meta
@@ -139,7 +139,7 @@ jobs:
     outputs:
       matrix: ${{steps.read_json.outputs.matrix}}
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - id: read_json
         run: echo "matrix=$(jq -c . ci/ldmx-sw-to-test.json)" >> $GITHUB_OUTPUT
 
@@ -157,7 +157,7 @@ jobs:
     steps:
     -
       name: Download Build Context for Test Script
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
     -
       name: Install just
       uses: extractions/setup-just@v3

.github/workflows/ldmx-sw-dependabot.yml

@@ -15,7 +15,7 @@ jobs:
       actions: write
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0
       - name: fetch latest releases

.github/workflows/mdbook.yml

@@ -33,7 +33,7 @@ jobs:
       MDBOOK_VERSION: 0.4.36
       MDBOOK_ADMONISH_VERSION: 1.14.0 # need to re-run mdbook-admonish install if updating version
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Install mdBook
         run: ./.github/install-mdbook
       - name: Setup Pages