Backport of client: cleanup leaked iptables rules into release/1.2.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #15407 to be assessed for backporting due to the inclusion of the label backport/1.2.x.

The below text is copied from the body of the original PR.

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This PR adds a secondary path for cleaning up iptables created for an allocation
when the normal CNI library fails to do so. This typically happens when the state
of the pause container is unexpected - e.g. deleted out of band from Nomad. Before,
the iptables rules would be leaked which could lead to unexpected nat routing
behavior later on (in addition to leaked resources). With this change, we scan
for the rules created on behalf of the allocation being GC'd and delete them.

Fixes #6385

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