ci: do not use latest sphinx/breathe releases for docs

Sphinx version 3.0.0 release recently break doc build, lock version
of sphinx to something compatible while we upgrade dependencies...

breathe v4.15.0 was just released and fixes some compatibility issues
with latest sphinx, the combo works, however with many warnings that we
still need to either fix or whitelist. This is temporary until we are
able to use those new versions.

Backport of #24109 and #24166

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>