Upgrade to Go 1.20

[bradfitz]

Tracking bug for updating to Go 1.20.

Notes:

https://tip.golang.org/doc/go1.20

Tree is rebased at https://github.com/tailscale/go/commits/tailscale.go1.20 ... even smaller than our Go 1.19 delta. (net/http stuff is in, mutex contention counters are in)

Going to switch our tree to tailscale/go@6a17f14 (Go 1.20rc3)

Then apply the https://github.com/tailscale/tailscale/issues?q=is%3Aissue+is%3Aopen+Go+1.20+label%3Afuture-go changes.

[bradfitz]

(We won't depend on any Go 1.20 features in the open source tree until Go 1.20 is officially out)

[bradfitz]

Instead of the metrics we'd previously added to Go, we need to update our internal mutex contention metrics to use this: golang/go#49881

[bradfitz]

tailscale/tailscale is updated, but I hit problems in the corp repo: golang/go#58159

[bradfitz]

Okay, we're now using Go 1.20 in both repos.

We'll hold on using Go 1.20 API until it's officially out.