[management,client] Default to NetBird's upload service when nothing is configured

The previous commit made a peer with no destination — no MDM override, no URL
named by the caller, nothing published by its management server — refuse to
upload and keep the bundle local unless it was enrolled with NetBird's cloud.
That closed the reported data-boundary concern, but it broke the default for
everyone who uploads a bundle as part of their day: a self-hosted user opening
a support ticket got a refusal where the command used to work.

Product decision (NetBird's, not the reporter's): the knob to keep bundles
inside your own infrastructure is what this branch provides, and it is enough.
The default stays the service NetBird runs, self-hosted included. An admin who
needs the bundles to stay in-house configures the destination; until then the
everyday flow keeps working.

So ResolveUploadURL drops the cloud check, the sentinel error and the
managementURL argument, and never fails:

    MDM  >  explicitly named URL  >  published by management  >  NetBird's service

Nothing observable changes for a deployment that configures nothing, which also
removes two edge cases the fail-closed default had: a peer still enrolled on the
legacy api.wiretrustee.com host would have been classified self-hosted and
refused, and an upgrade would have silently stopped uploads for self-hosted
deployments relying on them. The privilege gate is unaffected — a host other
than the default one still requires a privileged caller, so pointing the CLI
somewhere other than what management published needs root.

client/server/debug.go

@@ -48,11 +48,7 @@ func (s *Server) DebugBundle(callerCtx context.Context, req *proto.DebugBundleRe
 	// it is the operator of this deployment naming their own upload service, and
 	// the peer already trusts that server for its whole configuration. Only a
 	// URL the local caller named goes through requirePrivilegeForUploadURL above.
-	uploadURL, err := debug.ResolveUploadURL(req.GetUploadURL(), publishedUploadURL, managementURL)
-	if err != nil {
-		log.Errorf("cannot upload debug bundle: %v", err)
-		return &proto.DebugBundleResponse{Path: path, UploadFailureReason: err.Error()}, nil
-	}
+	uploadURL := debug.ResolveUploadURL(req.GetUploadURL(), publishedUploadURL)
 
 	// The upload runs without s.mutex held: it does network I/O to a possibly
 	// slow destination and must not block the other RPCs that take the lock. The