moving towards a sourmash 5.0 release - idle thoughts

[ctb]

we have a lot of "breaking" issues that are waiting for v5 - see the "5.0" tag.

it will probably take a few weeks, or even a month or two, of work to resolve all of these, unless we do a sprint. (should we do a sprint or jamboree of some kind?)

I was thinking of repeating what we did for 4.0, roughly -

• switching the stable branch over to 4.x maintenance, and releasing it as a "final" series of minor releases for 4.x (4.6.x or 4.7.x most likely), adding deprecations for features changed in 5.0 as we go;
• switching the latest branch over to 5.0 preparatory, removing features and switching defaults as we go;
• here the key is not to repeat previous mistakes and to keep merging things into latest (5.0) and then backporting to stable (4.x)
• think a bit about keeping 4.x and 5.x doc branches going simultaneously (per https://github.com/sourmash-bio/sourmash/issues/1016#issuecomment-779383987) and keep them on stable until 5.0 is out for a while?

I don't have any particular timeline in mind for an actual 5.0 release - probably not until November 2022 or later.

ref this discussion re releasing 4.0 - https://github.com/sourmash-bio/sourmash/issues/1016

@bluegenes @luizirber thoughts welcome.