[Task]: Consider supporting old clients for hard->soft blocks

[willdurand]

Description

This hard->soft transition is out of scope but still something AMO technically supports. Per https://docs.google.com/document/d/1JmGUPD9fZ8ri_xdF1zBwOXsQHfBX-51fV2HYbBDsFQg/edit?usp=sharing, we could use a "super-mega-unified stash" to support any clients:

{
  "blocked": [], 
  "softblocked": ["@addon:version"],
  "unblocked": ["@addon:version"]
}

Old clients not support soft-blocking would see the version as unblocked (because they ignore properties they don't recognize, and they don't recognize softblocked in the stash), and new clients (that support soft-blocking) will see the version as soft-blocked.

This works fine, except that the current AMO implementation generates both filters and stashes at the exact same time. In a previous iteration for #15014, it was possible to have a new soft filter generated, and a stash - leaving the version in the unblocked list of the stash only. That is a problem because new clients will see the version as "unblocked" and - because the data in the filter isn't newer than the stash (same timestamp) - these clients will consider the version as unblocked instead of soft-blocked.

We need to think about how we want to improve the situation for old clients, if possible. Otherwise, we'll need to document that old clients will never unblock a hard block turned into a soft block.

Acceptance Criteria

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1. support old clients

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• If I have identified that the work is specific to a repository, I have removed "repository:addons-server" or "repository:addons-frontend"

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