do not require version keys in plate/well/labels

[jo-mueller]

Fixes ome/ngff#309
Fixes ome/ngff#322
Mirrors ome/ngff#324

This PR removes the requirement for having a version key under the plate/well/labels objects. In a previous version, the version key was moved up under the ome key. I clarified the statement under the ome section a little so it's clearer that the version key MUST be provided there.

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[dstansby]

👍 for the change. I just wanted to check if these changes to the next version of the spec are being collected anywhere in a changelog for implementers to follow along?

At the same time as this change to the spec text, shouldn't the schemas be updated too?

[jo-mueller]

@dstansby thanks for the feedback. There is now a high-level changelog in the version_history.md document, I've also added this change there.

As for the schemas, I think the requirement for the version key in the schemas had already been dropped in 0.5 and making the appropriate change in the spec text had just been forgotten. So this is more of a cleanup than a change to bring schemas and spec back in sync.

[dstansby]

Thanks! Of course, I forgot that the schema/text were out of sync before this.