Enforce a strict deprecation deadline

[nezia1]

Posting this here so that this gets seen by everyone.

I would like to propose, as @llakala pointed out in #99, that we enforce a strict deprecation deadline as to not end up with unnecessarily increasing eval times. This would be ideal to do alongside moving all of our deprecations in one big file, akin to how a few projects (e.g. nvf) do it, so that it makes it more convenient.

This would of course be necessary to mention in the README, as more often than not, deprecations tend to stick around for indeterminate periods of time on Nix projects.

We could also enforce that every deprecation under the hypothetical file have an accompanying comment pointing out the date under which said deprecation would be removed (this would also be ideal if we could add it under the warning). This would make it so that we could even deprecate them automatically, perhaps with some CI/CD magic and clever text parsing.

I'm unsure about the dates, because that could depend from module to module, but I would like to discuss a rough estimate or what you folks think about the idea in general.