Drop support for delegating email validation (#13192)

* Drop support for delegating email validation

Delegating email validation to an IS is insecure (since it allows the owner of
the IS to do a password reset on your HS), and has long been deprecated. It
will now cause a config error at startup.

* Update unit test which checks for email verification

Give it an `email` config instead of a threepid delegate

* Remove unused method `requestEmailToken`

* Simplify config handling for email verification

Rather than an enum and a boolean, all we need here is a single bool, which
says whether we are or are not doing email verification.

* update docs

* changelog

* upgrade.md: fix typo

* update version number

this will be in 1.64, not 1.63

* update version number

this one too

tests/rest/client/test_register.py

@@ -592,9 +592,9 @@ class RegisterRestServletTestCase(unittest.HomeserverTestCase):
                 "require_at_registration": True,
             },
             "account_threepid_delegates": {
-                "email": "https://id_server",
                 "msisdn": "https://id_server",
             },
+            "email": {"notif_from": "Synapse <synapse@example.com>"},
         }
     )
     def test_advertised_flows_captcha_and_terms_and_3pids(self) -> None: