downgrading from 3.7.x to 3.6.x should be easier / supported

Created by: slimsag

Quoting my Slack message on this topic:

If a customer upgrades to v3.7.1, encounters some issues in the product (not necessarily related to search) they effectively cannot downgrade without suffering very bad search perf (unindexed) while we reindex everything back to the old v15 format.

Keegan suggested the following and estimates it would take about a day:

this is probably why zoekt didn’t delete before 🙂 We should probably make it automatic after 2 version increases. Then we tell admins to manually delete (very easy to do). [...] We already have the .trash dir, we could use that. If we include functionality to delete from the trash dir if disk space is low that may help?

I think we should do the above ASAP to make downgrading seamless.