Bump actions/checkout from 4 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Commits](actions/checkout@v4...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/push_gem.yml

@@ -26,7 +26,7 @@ jobs:
         with:
           egress-policy: audit
 
-      - uses: actions/checkout@0ad4b8fadaa221de15dcec353f45205ec38ea70b # v4.1.4
+      - uses: actions/checkout@1af3b93b6815bc44a9784bd300feb67ff0d1eeb3 # v6.0.0
       - name: Set up Ruby
         uses: ruby/setup-ruby@cacc9f1c0b3f4eb8a16a6bb0ed10897b43b9de49 # v1.176.0
         with:

.github/workflows/stable.yml

@@ -26,7 +26,7 @@ jobs:
           - truffleruby
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - run: rm Gemfile.lock
 

.github/workflows/unstable.yml

@@ -20,7 +20,7 @@ jobs:
           - truffleruby-head
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - uses: ruby/setup-ruby@v1
         with: