Revert "Revert "Drop support for delegating email validation (#13192)" (#13406)"

This reverts commit 98fb610.

Author: David Robertson

docs/upgrade.md

@@ -89,6 +89,25 @@ process, for example:
     dpkg -i matrix-synapse-py3_1.3.0+stretch1_amd64.deb
     ```
 
+# Upgrading to v1.66.0
+
+## Delegation of email validation no longer supported
+
+As of this version, Synapse no longer allows the tasks of verifying email address
+ownership, and password reset confirmation, to be delegated to an identity server.
+This removal was previously planned for Synapse 1.64.0, but was
+[delayed](https://github.com/matrix-org/synapse/issues/13421) until now to give
+homeserver administrators more notice of the change.
+
+To continue to allow users to add email addresses to their homeserver accounts,
+and perform password resets, make sure that Synapse is configured with a working
+email server in the [`email` configuration
+section](https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html#email)
+(including, at a minimum, a `notif_from` setting.)
+
+Specifying an `email` setting under `account_threepid_delegates` will now cause
+an error at startup.
+
 # Upgrading to v1.64.0
 
 ## Deprecation of the ability to delegate e-mail verification to identity servers