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/gemstash-ci.yml

@@ -58,7 +58,7 @@ jobs:
       GEMSTASH_SPEC_REDIS_SERVERS: redis://localhost:6379
       GEMSTASH_SPEC_MEMCACHED_SERVERS: localhost:11211
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           persist-credentials: false
       - name: Setup ruby

.github/workflows/lint.yml

@@ -9,7 +9,7 @@ jobs:
     env:
       BUNDLE_WITH: "linting"
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         persist-credentials: false
     - uses: ruby/setup-ruby@v1