Make Core Data migration test dynamic and future-proof

[mokagio]

See discussion in this PR

[mokagio]

@jkmassel @kean @crazytonyli @dcalhoun how do you feel about this?

If you think it's still worth keeping it the backlog, I'll shadow it in our Apple Core board in the Automattic org to avoid forgetting about this. Otherwise, we can close and move on.

[kean]

We should be able to safely remove the first 120+ models. They have a significant impact on the app size.

In the worst case scenario, someone will need to re-install the app.

[mokagio]

We should be able to safely remove the first 120+ models. They have a significant impact on the app size.

@kean sounds good to me, if you have the bandwidth, do you mind addressing this?

Based on that, maybe we can change the expectation on the migration tests to focus on expiration data rather than coverage? I.e. Does it really make sense to support migrating through years worth of versions? It might be a better tradeoff to accept a technical lack of migration support when in practice most users would be on relative recent versions, for which we'd have dedicated and easier to maintain tests?