timeout(1): Update STANDARDS section

Update the STANDARDS section for timeout(1) to
POSIX 1003.1-2024.

Update mandoc to be able to render the
new POSIX macro.

Reviewed by: bapt, bcr
Approved by: bapt
Obtained from: OpenBSD
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D45691