update ebpf dependency to 0.20.0

[mtfriesen]

Update to eBPF 0.20.0 and:

1. complete XDP must define its eBPF structs and helpers itself #613
2. remove dependency on local nuget package source for 0.20.0 on arm64, stop downloading the prerelease packages in prepare-machine.ps1 script, and complete validate onebranch-style builds for arm64, too #756

[mtfriesen]

Blocked on eBPF producing a viable arm64 release.

[mtfriesen]

Superseded and resolved by #828