[message] simplify method for allocating new message from pool (#7210)

This commit removes different `MessagePool` methods that can be used
to allocate a new message  and combines them into one `Allocate()`
method which uses `Message::Settings`. The `Message::Settings` is
also updated to provide new constructor initializing it with a given
message priority only.

tests/unit/test_message_queue.cpp

@@ -88,8 +88,8 @@ void TestMessageQueue(void)
 
     for (ot::Message *&msg : messages)
     {
-        msg = sMessagePool->New(ot::Message::kTypeIp6, 0);
-        VerifyOrQuit(msg != nullptr, "Message::New() failed");
+        msg = sMessagePool->Allocate(ot::Message::kTypeIp6);
+        VerifyOrQuit(msg != nullptr, "Message::Allocate() failed");
     }
 
     VerifyMessageQueueContent(messageQueue, 0);