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/gem_release.yaml

@@ -15,7 +15,7 @@ jobs:
     name: Build the gem
     runs-on: ubuntu-24.04
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Install Ruby
         uses: ruby/setup-ruby@v1
         with:

.github/workflows/unit_tests.yaml

@@ -22,7 +22,7 @@ jobs:
     outputs:
       ruby: ${{ steps.ruby.outputs.versions }}
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Install Ruby 3.4
         uses: ruby/setup-ruby@v1
         with:
@@ -43,7 +43,7 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - name: Checkout current PR
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Rspec checks
         uses: ruby/setup-ruby@v1