build(deps): 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

@@ -22,7 +22,7 @@ jobs:
           echo "$HOME/.elan/bin" >> "$GITHUB_PATH"
       - name: Restore cache
         id: cache-restore
-        uses: actions/cache/restore@v5
+        uses: actions/cache/restore@v6
         with:
           path: .lake
           key: lake-${{ runner.os }}-${{ runner.arch }}-${{ hashFiles('lean-toolchain') }}-${{ hashFiles('lake-manifest.json') }}-${{ github.sha }}
@@ -46,7 +46,7 @@ jobs:
 
       - name: Save project cache
         if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository
-        uses: actions/cache/save@v5
+        uses: actions/cache/save@v6
         with:
           path: .lake
           key: ${{ steps.cache-restore.outputs.cache-primary-key }}