Honor de.sorunome.msc2409.push_ephemeral flag in appservice handler

[weebl2000]

fixes #19927

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[anoadragon453]

Fixes a regression introduced in #19758.

That PR tried to maintain support for the old, unstable flag. But this change without the one in this PR meant that service.supports_ephemeral only applied to the new, stable config option. Thus, deployments on the old config option broke.

Seems we should have some unit tests which runs with an AS still using and expecting the old unstable identifier.

Thanks for catching this @weebl2000, and sorry about the disruption.

[weebl2000]

Updated changelog.d filename to reference the PR rather than the issue.