Remove usage of deprecated string-based API

[michalpasztamobica]

String-based APIs were marked deprecated and replaced by SocketAddress-based APIs in ARMmbed/mbed-os#11914.
This PR switches the example to the new API and removes the deprecated one.
It will only compile with the PR mentioned above or simply with mbed-os-5.15 or later (when the new SocketAddress-based API is available).

[michalpasztamobica]

@SeppoTakalo , @AnttiKauppila , @0xc0170 please review.
Note 1: This blocks ARMmbed/mbed-os#11942
Note 2: To get this compile I updated mbed-os.lib version, but not to any particular release, just to latest so this project gets the SocketAddress-based API. I hope this is ok?

[AnttiKauppila]

Why is this blocking Mbed OS?
I think the example needs to be updated after 5.15 release and only then modify the functions to have SocketAddress based function calls. And mbed-os.lib then points to 5.15 hash

[michalpasztamobica]

Unfortunately, mbed-os CI runs all example projects, see this comment and this CI failure report.

[0xc0170]

I think the example needs to be updated after 5.15 release and only then modify the functions to have SocketAddress based function calls. And mbed-os.lib then points to 5.15 hash

Isn't this applicable for the release branch ? This lands on master here (not 5.15 release branch). After 5.15 release, you can push this to 5.15 release branch in this example.

Note, will change once we change our release process in January, but for now should be fine.

[michalpasztamobica]

Update to mbed-os-5.15 added (in a separate commit).
@AnttiKauppila , @0xc0170 , please review.