util: Use std::chrono for time getters

Summary:
remove Boost posix_time usage from GetTime*

This is a backport of core#21110 [2/2]
https://github.com/bitcoin/bitcoin/pull/21110/commits/9266f7497f256d780178829e0f3a29ddaeb794ba

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11790