Upgrade libexpat to 2.7.4

[bcoconni]

Upgrade JSBSim local copy of libexpat from 2.7.0 to 2.7.4. This new version includes a number of bugs and security fixes (see libexpat changelog for details).

For the record, a JSBSim local copy of libexpat is used by the Python wheels from PyPI in order to support systems where libexpat is not installed (i.e Windows).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 25.28%. Comparing base (ab9037b) to head (c89fe49).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1388   +/-   ##
=======================================
  Coverage   25.28%   25.28%           
=======================================
  Files         169      169           
  Lines       18569    18569           
=======================================
  Hits         4696     4696           
  Misses      13873    13873           

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