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/ci.yml

@@ -80,7 +80,7 @@ jobs:
           echo "BUILD_CACHE_PREFIX_WITH_HASH=${{env.BUILD_CACHE_PREFIX}}-hash-${{hashFiles('mix.lock')}}" >> "$GITHUB_ENV"
 
       - name: Restore cached build artifacts
-        uses: actions/cache/restore@v5
+        uses: actions/cache/restore@v6
         with:
           path: |
             _build
@@ -115,7 +115,7 @@ jobs:
         # We always save to cache, even on failure, so that whatever artifacts
         # took a long time to generate are kept (looking at you, Dialyzer PLT)
         if: always()
-        uses: actions/cache/save@v5
+        uses: actions/cache/save@v6
         with:
           path: |
             _build