[BUG] Remove "Native POSIX" board support

[rexut]

Describe the bug
As notice by Zephyr upstream documentation, the board support for native_posix is deprecated in favour of native_sim, and will be removed in the Zephyr v4.2 release.

To Reproduce
See upstream documentation: https://docs.zephyrproject.org/latest/boards/native/native_posix/doc/index.html

Expected behavior
All test suites and samples should be revised to use only native_sim and/or native_sim/native/64.

Impact
Minor impact, the GH workflows should be running w/o failures.

Logs and console output
None.

Screenshots
None.

Development Environment (please complete the following information):
None.

Additional context
None.