VersionedSchema -> ObjectSchema

[jlowin]

Thanks for contributing to Prefect!

Please describe your work and make sure your PR:

• adds new tests (if appropriate)
• updates CHANGELOG.md (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

What does this PR change?

• Removes VersionedSchema. The intent of this class was to create an explicit way of managing schemas across various versions of Prefect, allowing any Prefect version to work with objects from any other version. Unfortunately, it required too much work. To implement cross-version serialization, every single object would have to be reissued for any version of Prefect that introduced a schema change. Instead, we take out the versioning features and continue to use this base class (renamed ObjectSchema) for its useful automatic-deserialization functionality. We leave the __version__ field in place as a useful bit of information
• Adds tests to make sure the current versions of Prefect can handle objects generated by old versions of Prefect (these should be updated appropriately)
• Prefect serialization now handle unknown fields gracefully (by ignoring them), ensuring that old versions of Prefect can handle objects generated by new versions of Prefect. Closes Make sure old versions of serializers ignore new fields #582

Why is this PR important?

Once flows are deployed, the version of Prefect used by the Flow may differ from versions of Prefect it communicates with. It's important that objects can be exchanged between the two versions no matter what.

[joshmeek]

Handling unknowns gracefully by just ignoring them is excellent life advice that extends far beyond the Prefect library.

[joshmeek]

LGTM, wait on @cicdw input for merging

[cicdw]

Made two absolutely trivial suggested changes (spelling), otherwise this looks good to me!

[jlowin]

Two spelling errors on "older"... can't be a coincidence.

My keyboard is out to get me.