Cop to disallow methods that use subtransactions

What does this MR do?

Adds a new cop that disallows use of common methods that use subtransactions in their implementation.

See #338730 (closed), also #338346 (closed) for surrounding context of why this is needed.

Also see !68576 (merged), where we disallow the direct use of subtransactions.

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How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

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