Clear-docker-cache: drop unsupported -a from podman volume prune

What does this MR do?

clear-docker-cache: drop unsupported -a from podman volume prune

The podman fallback added in !6755 (merged) ran podman volume prune -af, but podman's volume prune has no -a/--all flag (docker only added it in 23.0), so prune-volumes failed with:

Error: unknown shorthand flag: 'a' in -af

podman's volume prune already removes every volume not owned by a container, so -f alone is the correct equivalent of docker's --all. system prune -af is unaffected; podman supports -a there.

Closes #36934 (closed)

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com

Why was this MR needed?

As per the podman docs:

https://docs.podman.io/en/v5.1.2/markdown/podman-volume-prune.1.html

-f is all that is needed.

What's the best way to test this MR?

What are the relevant issue numbers?