package/ci: use Doxygen 1.8.15.

package/ci: use Doxygen 1.8.15.

Finally, a year later, a stable release I can use for testing on Travis.
On the other hand I'm furious because the old FTP got lost along with
all files it had and now the only possibility is to fetch infested crap
from sourceforge.

Also UGH WHY SO SLOW.