Remove references to the Ubuntu Xenial release as it is no longer supported by GitHub.

Author: bcoconni

.github/workflows/cpp-python-build.yml

@@ -584,7 +584,7 @@ jobs:
             ## Release files
             - Windows 64 bits
               - `JSBSim-${{ env.VERSION }}-setup.exe` (Windows installer)
-            - Ubuntu Xenial 16.04, Bionic 18.04 or Focal 20.04 - amd64
+            - Ubuntu Bionic 18.04 and Focal 20.04 - amd64
               - `*.deb` (Debian packages)
             - Python wheel packages can be installed from [PyPI](https://pypi.org/project/JSBSim/) with the command `pip install jsbsim` or `pip install --upgrade jsbsim` when JSBSim is already installed on your system.
             - Conda packages are available from [conda-forge](https://anaconda.org/conda-forge/jsbsim). To install this package with conda run: `conda install -c conda-forge jsbsim`