MAINTAINERS: Remove myself, add Ratan and Richard

I haven't written any code in this project, and I haven't been able
allocate much time to peer review either so it doesn't make any sense
for me to be a maintainer.

Richard and Ratan both have written code in PUM, been active in peer
review, and know a lot about the overall user management implementation
in OpenBMC.  Richard and Ratan will both provide timely and quality
feedback to PUM contributors, so it makes a lot of sense for them to
co-maintain PUM in place of Brad.

Change-Id: I72b9c471f2c42b4b962de4ecc040d6c8489ee21f
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
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