Revert the FlightGear network protocol to v24

[bcoconni]

The FlightGear ticket 2313 has introduced a change in the FlightGear network protocol which broke backward compatibility with the then current version 24. That forced us to support a new version of the protocol via the commit b42f423. Unfortunately the support of the new protocol did not work well (see issue #325) and the code to support both protocol versions 24 and 25 was ugly.

It was later discovered that the new network protocol was also breaking the interface between Simulink and FlightGear (see FlightGear ticket 2742) so the decision has been made by the FlightGear developers to drop the protocol version 25 and revert it back to 24.

The network protocol version 25 has never been part of an official FlightGear release so reverting it back to 24 is not breaking backward compatibility.

This PR reverts the commit b42f423 since the support of the protocol version 25 is now meaningless. This should fix the issue #325 in the process.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 141 lines in your changes missing coverage. Please review.

Project coverage is 24.78%. Comparing base (e6208a3) to head (3061255).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/input_output/FGOutputFG.cpp	0.00%	141 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1234      +/-   ##
==========================================
+ Coverage   24.76%   24.78%   +0.01%     
==========================================
  Files         170      170              
  Lines       19464    19450      -14     
==========================================
  Hits         4820     4820              
+ Misses      14644    14630      -14     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.