Add forget_forced_upon_leave capability as per MSC4267

[Johennes]

This adds the capability from https://github.com/matrix-org/matrix-spec-proposals/pull/4267.

Synapse PR: https://github.com/element-hq/synapse/pull/18196

Checklist

• [ ] Tests written for new code (and old code if feasible).
• [ ] New or updated public/exported symbols have accurate TSDoc documentation.
• [ ] Linter and other CI checks pass.
• [x] Sign-off given on the changes (see CONTRIBUTING.md).

[Johennes]

The naming confused me, have raised a point in https://github.com/matrix-org/matrix-spec-proposals/pull/4267/files#r1975272642

This is great, thank you. I literally just wanted to leave a comment saying that I'd be happy to receive suggestions for better naming of the capability. 😅

[t3chguy]

@Johennes looks like more concerns have been raised in the MSC re the naming, are you happy for this PR to wait until those are settled or do you desire this to land before FCP?

@richvdh also raised the point oob that the new symbol warrants some TSDoc

[Johennes]

Sorry, I should have put this back into draft state after getting approval. I'm fine with this lingering until the MSC lands (or is rejected). If this moves forward, I'll make sure to add the missing docs.

[github-actions[bot]]

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