MAINTAINERS: retire Boris from TLS maintainers

There's a steady stream of TLS changes and bugs. We need active
maintainers in this area, and Boris hasn't been participating
much in upstream work. Move him to CREDITS. While at it also
add Dave Watson there who was the author of the initial SW
implementation, AFAIU.

Link: https://patch.msgid.link/20250825155753.2178045-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>