Implement MSC3759: Leave event metadata for deactivated users

This issue has been migrated from [#12360](https://github.com/matrix-org/synapse/pull/12360).

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This PR implements [MSC3759](https://github.com/matrix-org/matrix-spec-proposals/pull/3759), which includes an extra key on the leave membership events sent when deactivating a user.

N.B. I couldn't find any tests that tested the existing behavior, so some pointers on how to write those tests would be good :)

Start with MSC3759 and the deactivation flow that emits leave membership events; the issue notes that existing behavior lacks dedicated tests, so locate the relevant event-generation tests or test entry point. Done means the leave events for deactivated users include the metadata required by the MSC and the behavior is covered by tests.

Written by the indexing model from the issue text.