Bump docker/build-push-action from 3 to 6

Bumps [docker/build-push-action](https://github.com/docker/build-push-action) from 3 to 6.
- [Release notes](https://github.com/docker/build-push-action/releases)
- [Commits](docker/build-push-action@v3...v6)

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

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/build_images.yaml

@@ -52,7 +52,7 @@ jobs:
         password: ${{ secrets.HARBOR_TOKEN }}
     - name: Build & push to staging
       if: ${{ github.ref != 'refs/heads/master' }}
-      uses: docker/build-push-action@v3
+      uses: docker/build-push-action@v6
       with:
         cache-from: type=gha
         cache-to: type=gha,mode=max
@@ -61,7 +61,7 @@ jobs:
         tags: "harbor.stfc.ac.uk/stfc-cloud-staging/${{ matrix.image }}:${{ needs.setup.outputs.sha }}"
     - name: Build & push to prod
       if: ${{ github.ref == 'refs/heads/master' }}
-      uses: docker/build-push-action@v3
+      uses: docker/build-push-action@v6
       with:
         push: true
         context: "{{defaultContext}}:${{ matrix.image }}"