Remove branch exceptions.

Bump pypa/cibuildwheel from 2.15 to 2.16

Fix the pkgconfig file (issue JSBSim-Team#770)

Fix the stable release process.

Improvements to the build of wheels for Linux.

- Set the flag `-fno-math-errno` so that the `errno` flag is not any longer being set when math functions are called.
- Exclude using Cython version `0.29.26` for building Python wheels as it fails building wheels for PyPy38.

Fix the scripts to pass the XML validation.

Remove references to the Ubuntu Xenial release as it is no longer sup…

…ported by GitHub.

Always set useDefault to false when calling SGPropertyNode::tie()

See issue JSBSim-Team#363: setting `useDefault` to true is equivalent to have uninitialized variables.