refactor: remove legacy package

[ainghazal]

Checklist

• [x] I have read the contribution guidelines
• [x] Iff you changed code related to services, or inter-service communication, make sure you update the diagrams in ARCHITECTURE.md.
• [x] Reference issue for this pull request:

Description

This commit deletes the legacy implementation that was still around under vpn/.

Practically all the functionality has been moved to the public pkg/tunnel package, which contains the public API now.

The test binary can now be found in the same path as before, cmd/minivpn, although the cli flags have changed. Behavior-wise, however, there are bits that I have not still ported (gvisor-based tun device). They will be ported on a per-need basis, but at least we're cleaning up the tree of the unused codebase for now.

I am also bumping go.mod and go.sum, minivpn now requires go 1.20.

[ainghazal]

added changes to support go 1.20, as preparation to use it as a dependency from probe-cli