server : use lambda instead of std::bind (#11507)

server : use lambda instead of std::bind (#11507)

This commit replaces the two usages of `std::bind` in favor of lambdas for
the callback functions for `callback_new_task` and
`callback_update_slots`.

The motivation for this changes is consistency with the rest of the code
in server.cpp (lambdas are used for all other callbacks/handlers). Also
lambdas are more readable (perhaps this is subjective) but also they are
recommended over `std::bind` in modern C++.

Ref: https://github.com/LithoCoders/dailycpp/blob/master/EffectiveModernC%2B%2B/chapter6/Item34_Prefer_lambdas_to_std::bind.md