MAINTAINERS: update SCTP maintainers

Vlad has stepped away from SCTP related duties.
Move him to CREDITS and add Xin Long.

Subsystem SCTP PROTOCOL
  Changes 237 / 629 (37%)
  Last activity: 2022-12-12
  Vlad Yasevich <vyasevich@gmail.com>:
  Neil Horman <nhorman@tuxdriver.com>:
    Author 20a785aa 2020-05-19 00:00:00 4
    Tags 20a785aa 2020-05-19 00:00:00 84
  Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>:
    Author 557fb586 2021-07-28 00:00:00 41
    Tags da05cecc 2022-12-12 00:00:00 197
  Top reviewers:
    [15]: lucien.xin@gmail.com
  INACTIVE MAINTAINER Vlad Yasevich <vyasevich@gmail.com>

Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>