Deprecate slirp4netns when gvisor-tap-vsock matures

[AkihiroSuda]

As gvisor-tap-vsock is now embedded in RootlessKit v3.0, I'm considering deprecating slirp4netns in favor of it so as to reduce package dependencies and unsafe/legacy C codes, but not right now.

The following PRs should be merged before deprecation:

• WIP: IPv6 support containers/gvisor-tap-vsock#59
• forwarder: propagate source IP containers/gvisor-tap-vsock#634