Update Doxygen file

[dstebila]

Fixes #1029

[xvzcf]

The Doxygen version in openquantumsafe/ci-ubuntu-focal-x86_64:latest (used by the stylecheck CI job) is still 1.8.17, which is probably the reason for the following warnings:

warning: ignoring unsupported tag 'PYTHON_DOCSTRING' at line 236, file docs/.Doxyfile
warning: ignoring unsupported tag 'NUM_PROC_THREADS' at line 474, file docs/.Doxyfile
warning: ignoring unsupported tag 'RESOLVE_UNNAMED_PARAMS' at line 544, file docs/.Doxyfile
warning: ignoring unsupported tag 'HTML_FORMULA_FORMAT' at line 1588, file docs/.Doxyfile
warning: ignoring unsupported tag 'DOT_UML_DETAILS' at line 2417, file docs/.Doxyfile
warning: ignoring unsupported tag 'DOT_WRAP_THRESHOLD' at line 2426, file docs/.Doxyfile
warning: argument 'FAIL_ON_WARNINGS' for option WARN_AS_ERROR is not a valid boolean value
Using the default: NO!

Works fine locally on macOS though, we could merge this in and open an issue in ci-containers to track the update there?

[baentsch]

CI docker image now contains doxygen of this version, so warnings are no longer emitted. If the logic is OK with @xvzcf I'm surely also OK with this.