[mqtt] Fix const-correctness for trigger constructors (#15093)

esphome/components/mqtt/mqtt_client.h

@@ -366,14 +366,14 @@ class MQTTJsonMessageTrigger : public Trigger<JsonObjectConst> {
 
 class MQTTConnectTrigger : public Trigger<bool> {
  public:
-  explicit MQTTConnectTrigger(MQTTClientComponent *&client) {
+  explicit MQTTConnectTrigger(MQTTClientComponent *client) {
     client->set_on_connect([this](bool session_present) { this->trigger(session_present); });
   }
 };
 
 class MQTTDisconnectTrigger : public Trigger<MQTTClientDisconnectReason> {
  public:
-  explicit MQTTDisconnectTrigger(MQTTClientComponent *&client) {
+  explicit MQTTDisconnectTrigger(MQTTClientComponent *client) {
     client->set_on_disconnect([this](MQTTClientDisconnectReason reason) { this->trigger(reason); });
   }
 };