Mbed OS 6.x deprecation warnings and MQTTClientMbedOs naming confusion

[panchalvatneu]

Hi,

I'm using this library with Mbed OS 6.17 and ran into two related issues:

1. Deprecation warnings from the use of Timer::read_ms() in MQTTmbed.h.
   Mbed OS 6 recommends using the chrono-based elapsed_time() API instead.

2. Some confusion around MQTTClientMbedOs.h: the header name suggests a
   MQTTClientMbedOs type, but the actual client type is MQTTClient. This led
   to build errors until I dug through the headers.

If this would be helpful, I’m happy to submit a small PR to:

• Update timing code to use elapsed_time() with version guards
• Improve documentation or comments to clarify correct usage

Just wanted to check if that would be welcome before opening a PR.

Thanks!