build(deps): bump docker/build-push-action from 4.0.0 to 6.9.0

Bumps [docker/build-push-action](https://github.com/docker/build-push-action) from 4.0.0 to 6.9.0.
- [Release notes](https://github.com/docker/build-push-action/releases)
- [Commits](docker/build-push-action@3b5e802...4f58ea7)

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

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

.github/workflows/ci-build.yml

@@ -443,7 +443,7 @@ jobs:
         images: harbor.stfc.ac.uk/datagateway/datagateway-api
 
     - name: ${{ github.event_name == 'push' && github.ref == 'refs/heads/main' && 'Build and push Docker image to Harbor' || 'Build Docker image' }}
-      uses: docker/build-push-action@3b5e8027fcad23fda98b2e3ac259d8d67585f671 # v4.0.0
+      uses: docker/build-push-action@4f58ea79222b3b9dc2c8bbdd6debcef730109a75 # v6.9.0
       with:
         context: .
         push: ${{ github.event_name == 'push' && github.ref == 'refs/heads/main' }}