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

@@ -8,7 +8,7 @@ jobs:
         ruby: [ 2.6, 2.7, '3.0', 3.1, 3.2, 3.3, 3.4, ruby-head, truffleruby-head ]
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
@@ -19,8 +19,8 @@ jobs:
     name: Check ruby/spec still passes with MSpec changes
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
+    - uses: actions/checkout@v6
       with:
         repository: ruby/spec
         path: rubyspec