Revert "Pin zun jobs to Docker 20"

This reverts commit 94a74f58.

Reason for revert: Zun/Kuryr-libnetwork has switched to "local" scope which doesn't require docker 20 anymore. It should work for latest docker version. Related patches:
https://review.opendev.org/c/openstack/zun/+/903884

Change-Id: Ieb545ae5a5917322f599728587c3f04ea8356126