ci: Purge old untagged container images

We were piling them up for too long, up to a thousand of them. There is
simple mitigation, though, just run the delete-package-versions action
on them.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

.github/workflows/next.yml

@@ -150,3 +150,18 @@ jobs:
             DEBIAN_TAG=${{ env.DEBIAN_TAG }}
           outputs: type=registry,rewrite-timestamp=true
           tags: ghcr.io/siemens/kas/kas-isar:next
+
+      - name: Purge oldest untagged kas images
+        uses: actions/delete-package-versions@v4
+        with:
+          package-name: kas
+          package-type: container
+          min-versions-to-keep: 10
+          delete-only-untagged-versions: true
+      - name: Purge oldest untagged kas-isar images
+        uses: actions/delete-package-versions@v4
+        with:
+          package-name: kas-isar
+          package-type: container
+          min-versions-to-keep: 10
+          delete-only-untagged-versions: true