resource: use Patch in APIFinalizer

[sttts]

Description of your changes

Calling Update on a non-unstructured object will potentially lead to data-loss because it is not guaranteed that the object is a complete representation of all the fields stored in the apiserver, e.g. because of version skew between client and Crossplane.

I have:

• [ ] is that type cast safe?
• [ ] do we have enough test coverage to confirm the safety of this change?
• [ ] maybe run proof PR in crossplane
• [x] Read and followed Crossplane's contribution process.
• [x] Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

Via Crossplane test suite, aka unit tests and e2e tests.

[sttts]

Actually, this can lose changes that were made before calling the finalizer code, i.e. there is no way around comparing to a fresh object :-/

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