cmd,tsnet,internal/client: create internal shim to deprecated control plane API

Even after we remove the deprecated API, we will want to maintain a minimal
API for internal use, in order to avoid importing the external
tailscale.com/client/tailscale/v2 package. This shim exposes only the necessary
parts of the deprecated API for internal use, which gains us the following:

1. It removes deprecation warnings for internal use of the API.
2. It gives us an inventory of which parts we will want to keep for internal use.

Updates tailscale/corp#22748

Signed-off-by: Percy Wegmann <percy@tailscale.com>

cmd/tailscale/cli/up.go

@@ -27,8 +27,8 @@ import (
 	"github.com/peterbourgon/ff/v3/ffcli"
 	qrcode "github.com/skip2/go-qrcode"
 	"golang.org/x/oauth2/clientcredentials"
-	"tailscale.com/client/tailscale"
 	"tailscale.com/health/healthmsg"
+	"tailscale.com/internal/client/tailscale"
 	"tailscale.com/ipn"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/net/netutil"
@@ -1097,12 +1097,6 @@ func exitNodeIP(p *ipn.Prefs, st *ipnstate.Status) (ip netip.Addr) {
 	return
 }
 
-func init() {
-	// Required to use our client API. We're fine with the instability since the
-	// client lives in the same repo as this code.
-	tailscale.I_Acknowledge_This_API_Is_Unstable = true
-}
-
 // resolveAuthKey either returns v unchanged (in the common case) or, if it
 // starts with "tskey-client-" (as Tailscale OAuth secrets do) parses it like
 //