ci: bump actions/cache from 5 to 6

Bumps [actions/cache](https://github.com/actions/cache) from 5 to 6.
- [Release notes](https://github.com/actions/cache/releases)
- [Changelog](https://github.com/actions/cache/blob/main/RELEASES.md)
- [Commits](actions/cache@v5...v6)

---
updated-dependencies:
- dependency-name: actions/cache
  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

@@ -77,7 +77,7 @@ jobs:
         uses: actions/checkout@v7
 
       - name: Cache native build (CMake)
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           path: |
             build/native/gbmahjong

.github/workflows/test.yml

@@ -66,7 +66,7 @@ jobs:
         run: chmod +x ./gradlew
 
       - name: Cache native build (CMake)
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           path: |
             build/native/gbmahjong