fix doxygen version on readthedocs

4 jobs for readthedocs in 31 seconds (queued for 1 second)
Status Job ID Name Coverage
  Style
passed #30848
integration
checkpatch

00:00:30

passed #30847
integration
repoquality

00:00:03

 
  Build
canceled #30849
make:generic

00:00:00

canceled #30850
knl
make:knl