Warn about old entity spec version in form draft update

[ktuite]

Closes getodk/central#816

What has been done to verify that this works as intended?

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Does this change require updates to the API documentation? If so, please update docs/api.yaml as part of this PR.

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[ktuite]

Hmm this is actually kind of complicated because of how similar uploading a new draft version and duplicating the published version into a draft are. And it doesn't align with the existing duplicate schema check because the schema could stay the same but the entities version could still differ.

One idea is to not handle this case: If you upload a new version of an existing form that has an old entities xform spec, fine.