[CI:DOCS] Restore rootless-cni-infra contrib info

[TomSweeneyRedHat]

PR #9423 removed the contrib/rootless-cni-infra directory and
it's files and should not have. The information in that tree
is required for Podman v3.0 and earlier users, and primarily
those in RHEL 8.4.0 and 8.3.1.

This PR puts those files back and adds a note to the README.md
noting the changes are not needed for versions after Podman v3.0.

Signed-off-by: TomSweeneyRedHat tsweeney@redhat.com

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[mheon]

NACK. This is unnecessary. We are pointing people at the long-term support branches, not master, for the image. We should not continue to maintain this readme and image.

[TomSweeneyRedHat]

I'll buy the NACK when you show me where we're pointing people to find this info.... Someone running into this on Podman 2.2 I don't think will find it unless it's on master.

[mheon]

We point them at the specific release branch. From our 2.2.1 RHEL branch - 4007554 points people to https://github.com/containers/podman/blob/v2.2.1-rhel/contrib/rootless-cni-infra/ - that's not master, that's the v2.2.1-rhel branch. We did this deliberately to ensure they pulled a version of the infra image that matched their Podman. v3.0.1 is the same.