MAINTAINERS: change bridge maintainers

I haven't been doing reviews only but not active development on bridge
code for several years. Roopa and Nikolay have been doing most of
the new features and have agreed to take over as new co-maintainers.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>