Update nng version to v1.11, the lastest stable version

[aurebidart]

Fixes: #

About

This PR updates the NNG dependency used by Project AirSim from version v1.5.2 to v1.11.
The update is applied via the ExternalProject_Add(nng-external) directive in the root CMakeLists.txt, ensuring consistent builds with this specific version of the library.

How Has This Been Tested?

• Performed clean builds on both Linux and Windows, removing the previous NNG folders (_deps/nng-*) to force a fresh clone and build of the specified version.
• Verified that the correct version was used by running:

  git describe --tags

inside build\_deps\nng\src\nng-external\, which returned:

v1.11

• Recompiled the entire project on both platforms without issues.
• Loaded a simulation scene and verified that topic-based communication and services relying on NNG performed as expected in both environments.

Screenshots and videos (if appropriate):

N/A