cmd/tailscale/cli,ipn: mention available update in "tailscale status" (#9205)

Cache the last `ClientVersion` value that was received from coordination
server and pass it in the localapi `/status` response.
When running `tailscale status`, print a message if `RunningAsLatest` is
`false`.

Updates #6907

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

ipn/ipnstate/ipnstate.go

@@ -71,6 +71,8 @@ type Status struct {
 
 	Peer map[key.NodePublic]*PeerStatus
 	User map[tailcfg.UserID]tailcfg.UserProfile
+
+	ClientVersion *tailcfg.ClientVersion
 }
 
 // TKAKey describes a key trusted by network lock.