Deprecate approvers and approval_group fields

What does this MR do and why?

Describe in detail what your merge request does and why.

This adds some clarity around the previously deprecated fields for the merge approvals API. Pertinent details are in #30858 (comment 1175550826) - but the fields are already documented as deprecated in code. This just makes it more clear in the example with some additional notes and then also provides an actual deprecation notice.

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Related to #388311 (closed)