Mark Mixxx util function deprecated when they shouldn't be used anymore.

[acolombier]

A few Mixxx util function were flagged as deprecated on recent PR reviews and the request was made to use the std lib instead. In order to help with the DevX, it would be great if the Mixxx Core Team could take the time to mark all utils functions which shouldn't be used anymore as deprecated to prevent unnecessary effort, and document which is the preferred replacement for it. (eg mixxx::Duration -> std::chrono::duration)