Deprecate slirp4netns when gvisor-tap-vsock matures

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:
- https://github.com/containers/gvisor-tap-vsock/pull/59
- https://github.com/containers/gvisor-tap-vsock/pull/634