ci: doc: upgrade to Doxygen 1.9.1

Upgrade to Doxygen version 1.9.1. Package is downloaded directly from
the Doxygen official site, since it is not yet part of the latest Ubuntu
LTS. Note that libclang1-9 and libclang-cpp9 are runtime dependencies
required by Doxygen.

NOTE: Documentation can still be built with older Doxygen versions (e.g.
1.8.17 shipped with latest Ubuntu LTS), however, the template used in
upcoming patches claims to work better with Doxygen 1.9.1 or 1.9.2.
Using 1.9.1 as theme v1.6.0 has some issues on mobile view when using
Doxygen 1.9.2, see
https://github.com/jothepro/doxygen-awesome-css/issues/47

.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>