Update doxygen CI to track newer doxygen
Doxygen on Homebrew is currently 1.8.15, which is incompatible with some of the documentation written with 1.8.13.
The CI build should switch to using a more up-to-date doxygen.
Doxygen on Homebrew is currently 1.8.15, which is incompatible with some of the documentation written with 1.8.13.
The CI build should switch to using a more up-to-date doxygen.