maint: add attributes to two functions without side effects

maint: add attributes to two functions without side effects

* src/date.c (res_width): This function computes its result solely
from the value of its parameter and qualifies for the const attribute.
* src/tee.c (get_next_out): This function has no side effect and
qualifies for the pure attribute.
* THANKS.in: Remove duplicate now that author has a commit in the repo.

Those two functions were flagged by GCC 12.3.0,
though not by GCC 13.2.1.