Retain the "entity" field in PackageCompendiumData so that worlds which are migrated to v9 can (potentially) be reverse migrated back to v8.

[Fyorl]

Originally in GitLab by @Fyorl

Originally reported by KaKaRoTo#1337 https://discord.com/channels/170995199584108546/872553750877835264/905941392159834163

Short description of bug: World will disappear from Foundry setup when downgrading from v9 to 0.8.x
Details:
Reporting here as suggested by @cswendrowski so it can be fixed before stable.
If you upgrade to v9 (for testing purposes or because it's now stable), the world.json gets migrated where compendium packs get the entity field changed into type. This means that downgrading back to 0.8.x will cause the world to completely disappear from the setup page.
Error logs show that Foundry rejects the package because of the missing required field entity in CompendiumData.
While DB migration could cause a world to misbehave, and downgrades aren't technically supported, it is a common use case that people need to revert (broken modules, or game breaking bugs, or other) and not having the world completely disappear could help prevent a lot of tech support requests and user headaches.

[Fyorl]

Originally in GitLab by @Fyorl

I think we recommend that people take backups of their worlds before updating foundry versions so those worlds can be restored if they need to revert, so I'm not sure if we should be making allowances for these sorts of cases. If not though, it should be reasonably easy to retain the entity field in the CompendiumData and not delete it.

[Fyorl]

Originally in GitLab by @kakaroto

This doesn't look like it was fixed. I think the world.json will still not have the entity field if it's not part of the pack's schema, when the manifest gets re-written.

A user reported the same issue today and after I checked their world.json, it didn't show entity.

This was marked fixed on v9 Dev 2, but the user only had access to v9 Testing 1 (9.233). Please re-open and re-scope it before stable.

Thanks.

[Fyorl]

Originally in GitLab by @anathemamask

Still occurring in v9t1 world pack definitions.