ipn/ipnlocal: drop h2c package, use net/http's support

In Dec 2021 in d3d503d997 I had grand plans to make exit node DNS
cheaper by using HTTP/2 over PeerAPI, at least on some platforms. I
only did server-side support though and never made it to the client.

In the ~4 years since, some things have happened:

* Go 1.24 got support for http.Protocols (https://pkg.go.dev/net/http#Protocols)
  and doing UnencryptedHTTP2 ("HTTP2 with prior knowledge")
* The old h2c upgrade mechanism was deprecated; see https://github.com/golang/go/issues/63565
  and https://github.com/golang/go/issues/67816
* Go plans to deprecate x/net/http2 and move everything to the standard library.

So this drops our use of the x/net/http2/h2c package and instead
enables h2c (on all platforms now) using the standard library.

This does mean we lose the deprecated h2c Upgrade support, but that's
fine.

If/when we do the h2c client support for ExitDNS, we'll have to probe
the peer to see whether it supports it. Or have it reply with a header
saying that future requests can us h2c. (It's tempting to use capver,
but maybe people will disable that support anyway, so we should
discover it at runtime instead.)

Also do the same in the sessionrecording package.

Updates #17305

Change-Id: If323f5ef32486effb18ed836888aa05c0efb701e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tsnet/depaware.txt

@@ -331,8 +331,7 @@ tailscale.com/tsnet dependencies: (generated by github.com/tailscale/depaware)
         golang.org/x/net/dns/dnsmessage                              from net+
         golang.org/x/net/http/httpguts                               from golang.org/x/net/http2+
         golang.org/x/net/http/httpproxy                              from net/http+
-        golang.org/x/net/http2                                       from golang.org/x/net/http2/h2c+
- LDW    golang.org/x/net/http2/h2c                                   from tailscale.com/ipn/ipnlocal
+        golang.org/x/net/http2                                       from tailscale.com/control/controlclient+
         golang.org/x/net/http2/hpack                                 from golang.org/x/net/http2+
         golang.org/x/net/icmp                                        from github.com/prometheus-community/pro-bing+
         golang.org/x/net/idna                                        from golang.org/x/net/http/httpguts+