MAINTAINERS: change reiserfs status to obsolete

Reiserfs file system is no longer supported and is going to be removed
in 2025 as stated in commit eb103a51

 ("reiserfs: Deprecate reiserfs").

Signed-off-by: Piotr Siminski <piotr.siminski@globallogic.com>
Reviewed-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20230720115445.15583-1-piotr.siminski@globallogic.com>