Merge pull request #296 from redpanda-data/fix-gha-cache-deprecation

gha: update buildkit to v0.21

Author: sbocinec

.github/workflows/docker-image.yml

@@ -26,7 +26,7 @@ jobs:
       - uses: docker/setup-buildx-action@v3
         with:
           driver-opts: |
-            image=moby/buildkit:v0.15.2
+            image=moby/buildkit:v0.21.1
             network=host
       - name: Set build date
         run: |