readme: referencing old example

This library is used by various applications or examples. The readme refernces unsupported example:
> For an example application, please see https://github.com/ARMmbed/ntp-client-example.

I found out while I was suprised this library has travis job set-up. However, the example fetches is dated back to mbed-os-5.3. I tried to migrate the example to latest Mbed OS with tools and updated compiler but it fails to build as we refactored networking in 6.x version. To unblock cloud examples being updated with CMake, I removed travis from here for now as this library will be tested in the cloud examples that use this library.

Also readme references Mbed OS 5.x version, shall we update this to bring to Mbed OS 6.x ?

Should we remove the reference to ntp-client-example and mark that example as deprecated. Or what are our options?

cc @ARMmbed/mbed-os-connectivity

Start with the README references to ntp-client-example and Mbed OS 5.x. Check whether the example is still supported and confirm the current Mbed OS version used by cloud examples; done means agreeing on and applying the updated or removed references, with the example marked deprecated if that is the chosen option.

Written by the indexing model from the issue text.