CREDITS, MAINTAINERS, docs/process/howto: Update man-pages' maintainer

Michael retired from maintaining the Linux man-pages project in 2021.
See commit 06e72cb19c74d3b1d661609c698ee26d7b6e4d7e ("CONTRIBUTING,
README, lsm: Remove mtk as maintainer") in the Linux man-pages
repository.

Add him to CREDITS.

Reported-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Link: <https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=06e72cb19c74d3b1d661609c698ee26d7b6e4d7e

>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Michael Kerrisk <mtk@man7.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240101124242.8059-2-alx@kernel.org