bin: update docker-prune script and playbook to use nerdctl

[anders-elastisys]

What this PR does / why we need it: Clusters no longer use docker for their container runtime, hence the prune-docker script/playbook no longer works. This PR replaces docker with nerdctl instead, which is installed in newer clusters.

Which issue this PR fixes: fixes https://github.com/elastisys/compliantkubernetes-kubespray/issues/306

Public facing documentation PR (if applicable)

Special notes for reviewer:

Checklist:

• [x] Proper commit message prefix on all commits
• [ ] Updated the public facing documentation
• Is this changeset backwards compatible for existing clusters? Applying:
  • [ ] is completely transparent, will not impact the workload in any way.
  • [ ] requires running a migration script.
  • [ ] requires draining and/or replacing nodes.
  • [ ] will break the cluster. I.e. full cluster migration is required.