MAINTAINERS: mailmap: Update Stanislav's email address

Moving to personal address for upstream work.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20240612225334.41869-1-sdf@google.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>