Refactor and adapt to C++20

[kostasrim]

There are large parts in the codebase that can leverage:

• concepts
• ranges
• coroutines -> any state machine mambo jumpo spaghetti code can be rewritten imperatively (no fancy callback or continuation style .then().then() noise).

@dranikpg had the liberty to start this effort #6403 but as this is something I longed for, I open this EPIC for us to keep track of the changes and the TODO/WANT-TO's.

*** IMPORTANT ***
This is a refactoring task with no really tangible value other than the subjective (arguably) code simplicity and expressiveness.

P.S I leave this as a background task to incrementally improve and adapt the codebase. On my side, I will only do this when I have minutes to spare :)

P.S.2 Write here anything you wish or want to see changed.