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/build.yml

@@ -11,11 +11,11 @@ jobs:
         working-directory: jspecify-reference-checker
     steps:
       - name: Check out jspecify-reference checker
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           path: jspecify-reference-checker
       - name: Check out jspecify
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           repository: jspecify/jspecify
           path: jspecify
@@ -47,7 +47,7 @@ jobs:
     if: github.repository == 'jspecify/jspecify-reference-checker' && github.ref_name == 'main' && github.event_name == 'push'
     steps:
     - name: Check out the code
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
     - name: Set up Java
       uses: actions/setup-java@v5
       with:

.github/workflows/update-conformance-test-reports.yml

@@ -25,13 +25,13 @@ jobs:
           distribution: temurin
           java-version: 17
       - name: Check out JSpecify
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           repository: jspecify/jspecify
           ref: main
           path: jspecify
       - name: Check out the Reference Checker
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           fetch-depth: 0
           path: reference-checker