Announce deprecation of `account_threepid_delegates.email`

[richvdh]

account_threepid_delegates.email was removed in #13192 (v1.64.0rc1) but that was reverted in #13406 (v1.64.0rc2).

As part of the release announcement for 1.64.0 final, I would like us to give an unambiguous indication of when it will be removed.

Initially we had hoped to do this in 1.65, but feedback is that that is too soon.

[richvdh]

I am inclined to compromise on 1.66.

[richvdh]

We've agreed as a team to target 1.66.0. We should ensure that this is clearly announced in the release announcement.

[DMRobertson]

We should ensure that this is clearly announced in the release announcement.

See https://matrix.org/blog/2022/08/03/synapse-1-64-released#delegating-email-verification-is-now-deprecated .