net/netcheck: adjust HTTPS latency check to connection time and avoid data race

The go-httpstat package has a data race when used with connections that
are performing happy-eyeballs connection setups as we are in the DERP
client. There is a long-stale PR upstream to address this, however
revisiting the purpose of this code suggests we don't really need
httpstat here.

The code populates a latency table that may be used to compare to STUN
latency, which is a lightweight RTT check. Switching out the reported
timing here to simply the request HTTP request RTT avoids the
problematic package.

Fixes tailscale/corp#25095

Signed-off-by: James Tucker <james@tailscale.com>

cmd/tailscale/depaware.txt

@@ -58,7 +58,6 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
    L 💣 github.com/tailscale/netlink                                 from tailscale.com/util/linuxfw
    L 💣 github.com/tailscale/netlink/nl                              from github.com/tailscale/netlink
         github.com/tailscale/web-client-prebuilt                     from tailscale.com/client/web
-        github.com/tcnksm/go-httpstat                                from tailscale.com/net/netcheck
         github.com/toqueteos/webbrowser                              from tailscale.com/cmd/tailscale/cli
    L    github.com/vishvananda/netns                                 from github.com/tailscale/netlink+
         github.com/x448/float16                                      from github.com/fxamacker/cbor/v2
@@ -306,7 +305,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         net                                                          from crypto/tls+
         net/http                                                     from expvar+
         net/http/cgi                                                 from tailscale.com/cmd/tailscale/cli
-        net/http/httptrace                                           from github.com/tcnksm/go-httpstat+
+        net/http/httptrace                                           from golang.org/x/net/http2+
         net/http/httputil                                            from tailscale.com/client/web+
         net/http/internal                                            from net/http+
         net/netip                                                    from go4.org/netipx+