Bump actions/checkout from 5 to 6 (#810)

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>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/build_docs.yml

@@ -28,7 +28,7 @@
 
      steps:
        - name: Checkout repo
-         uses: actions/checkout@v5
+         uses: actions/checkout@v6
          with:
            fetch-depth: 0 # Fetch all history for all branches and tags.
 

.github/workflows/release_and_publish.yml

@@ -28,7 +28,7 @@
 
      steps:
        - name: Checkout repo
-         uses: actions/checkout@v5
+         uses: actions/checkout@v6
          with:
            fetch-depth: 0 # Fetch all history for all branches and tags.
 

.github/workflows/reverse.yml

@@ -16,7 +16,7 @@ jobs:
     name: Reverse dependency testing
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0
 

.github/workflows/unittests.yml

@@ -43,7 +43,7 @@ jobs:
 
     steps:
       - name: Checkout repo
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           fetch-depth: 0 # Fetch all history for all branches and tags.