Replace deprecated ASIO functions and raise minimum Boost version

[ranisalt]

Pull Request Prelude

• [x] I have followed proper The Forgotten Server code styling.
• [x] I have read and understood the contribution guidelines before making this PR.
• [x] I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

Enables BOOST_ASIO_NO_DEPRECATED and fixes code that depends on deprecated facilities. While at that, bump the minimum required version of Boost to 1.71 which is the version shipped with the oldest Ubuntu release still supported (20.04 LTS)

libboost-dev for Ubuntu 20.04