doc: bump breathe>=4.21.0

Breathe 4.21.0 brings a few interesting features that improve the
generated documentation:

* A new `separate_member_pages` config option that patches issues in the
  Doxygen XML generated when SEPARATE_MEMBER_PAGES option is YES.
* A new rST verbatim mode that allows producing inline elements.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>