Deprecation schedule for minor (breaking) language changes

[WardBrian]

Rendered version: https://github.com/WardBrian/design-docs/blob/deprecation-schedule/designs/0031-language-deprecation-schedule.md

Prior discussion on forums (largely replicated in the doc): https://discourse.mc-stan.org/t/a-deprecation-schedule-for-stan/24228

[bob-carpenter]

The main contentful comment here is that we should commit to 3 minor releases or 1 major release for minor deprecations. We should also say anything that's not a minor deprecation as defined here will require a major version bump.

I hope you appreciate the more nitpicky comments rather than finding them annoying.

[WardBrian]

I'd like to ask for any more feedback on this. We will be discussing it during Thursday's language meeting and then merging unless there are objections

[bob-carpenter]

Looks good. Thanks!