Bump docker/build-push-action from 6 to 7

Bumps [docker/build-push-action](https://github.com/docker/build-push-action) from 6 to 7.
- [Release notes](https://github.com/docker/build-push-action/releases)
- [Commits](https://github.com/docker/build-push-action/compare/v6...v7)

---
updated-dependencies:
- dependency-name: docker/build-push-action
  dependency-version: '7'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/ci.yml

@@ -113,7 +113,7 @@ jobs:
 
       - id: build
         name: Build (and push by digest on master)
-        uses: docker/build-push-action@v6
+        uses: docker/build-push-action@v7
         with:
           context: .
           platforms: ${{ matrix.platform }}

package.json

@@ -14,7 +14,7 @@
     "autoprefixer": "^10.4.0",
     "bootstrap": "^5.3.8",
     "clipboard": "^2.0.8",
-    "cssnano": "^9.2.2",
+    "cssnano": "^5.0.13",
     "easymde": "^2.21.0",
     "fullcalendar": "^5.10.1",
     "gulp-concat": "^2.6.1",