ci(deps): bump actions/cache from 4 to 6

Bumps [actions/cache](https://github.com/actions/cache) from 4 to 6.
- [Release notes](https://github.com/actions/cache/releases)
- [Changelog](https://github.com/actions/cache/blob/main/RELEASES.md)
- [Commits](actions/cache@v4...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/copilot-setup-steps.yml

@@ -48,7 +48,7 @@ jobs:
 
       - name: Cache Playwright browsers
         if: ${{ hashFiles('playwright.config.*') != '' || hashFiles('.playwright-mcp/**') != '' }}
-        uses: actions/cache@v4
+        uses: actions/cache@v6
         with:
           path: ~/.cache/ms-playwright
           key: ${{ runner.os }}-playwright-${{ hashFiles('playwright.config.*', '.playwright-mcp/**') }}
@@ -107,7 +107,7 @@ jobs:
 
       # ---------- Optional caches ----------
       - name: Cache Next.js build artifacts
-        uses: actions/cache@v4
+        uses: actions/cache@v6
         with:
           path: |
             ~/.npm