Don't send unstable signing key update EDUs

[velikopter]

I think 5 years is enough.

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[reivilibre]

Thank you very much, always nice to get a cleanup.

Some archaeology notes:

• Stable version introduced in: https://github.com/matrix-org/synapse/pull/10520 (Synapse v1.50.0, 2022 January)
• Unstable version introduced in: https://github.com/matrix-org/synapse/pull/5727 (Synapse v1.6.0, 2019 November)
• Part of MSC1756

[reivilibre]

Now each entry only produces only one EDU, need to update the 'budgeting' logic on R668 and R691 to compensate.

There are also a bunch of stale comments to remove. :)

[velikopter]

I don't see any budgeting logic on R691..

[reivilibre]

sorry for being elusive, I think I meant

            if (
                is_cross_signing_key_update
                and user_id not in cross_signing_keys_by_user
            ):
                # This will give rise to 2 device updates.
                # If we don't have the budget, stop here!
                if remaining_length_budget < 2:
                    break

I think that full block can come out; if we updated this directly we'd change the 2nd if to if remaining_length_budget < 1: BUT we already have assert remaining_length_budget >= 0 earlier, so there is no time it would hit this anymore

[reivilibre]

Thank you!