Remove legacy MSC3861 auth delegation support (#19895)

Modern 'MAS integration' (as we call it now) is, of course, preserved.

Fixes: #19549

---------

Signed-off-by: Olivier 'reivilibre <oliverw@matrix.org>

docs/admin_api/user_admin_api.md

@@ -227,7 +227,7 @@ The following parameters should be set in the URL:
 - `name` - Is optional and filters to only return users with user ID localparts
   **or** displaynames that contain this value.
 - `guests` - string representing a bool - Is optional and if `false` will **exclude** guest users.
-  Defaults to `true` to include guest users. This parameter is not supported when MSC3861 is enabled. [See #15582](https://github.com/matrix-org/synapse/pull/15582)
+  Defaults to `true` to include guest users. This parameter is not supported when Matrix Authentication Service integration is enabled. [See #15582](https://github.com/matrix-org/synapse/pull/15582)
 - `admins` - Optional flag to filter admins. If `true`, only admins are queried. If `false`, admins are excluded from
   the query. When the flag is absent (the default), **both** admins and non-admins are included in the search results.
 - `deactivated` - string representing a bool - Is optional and if `true` will **include** deactivated users.
@@ -444,7 +444,7 @@ To unsuspend a user, use the same endpoint with a body of:
 
 ## Reset password
 
-**Note:** This API is disabled when MSC3861 is enabled. [See #15582](https://github.com/matrix-org/synapse/pull/15582)
+**Note:** This API is disabled when Matrix Authentication Service integration is enabled. Use the [MAS Admin API](https://element-hq.github.io/matrix-authentication-service/topics/admin-api.html) or [the MAS CLI](https://element-hq.github.io/matrix-authentication-service/reference/cli/manage.html#manage-set-password) instead. An easy way to make use of this functionality is provided by [Element Admin](https://element.io/en/server-suite/admin) as part of [ESS](https://element.io/en/server-suite) Community and Pro.
 
 Changes the password of another user. This will automatically log the user out of all their devices.
 
@@ -469,7 +469,7 @@ The parameter `logout_devices` is optional and defaults to `true`.
 
 ## Get whether a user is a server administrator or not
 
-**Note:** This API is disabled when MSC3861 is enabled. [See #15582](https://github.com/matrix-org/synapse/pull/15582)
+**Note:** This API is disabled when Matrix Authentication Service integration is enabled. Use the [MAS Admin API](https://element-hq.github.io/matrix-authentication-service/topics/admin-api.html) instead. An easy way to make use of this functionality is provided by [Element Admin](https://element.io/en/server-suite/admin) as part of [ESS](https://element.io/en/server-suite) Community and Pro.
 
 The api is:
 
@@ -488,7 +488,7 @@ A response body like the following is returned:
 
 ## Change whether a user is a server administrator or not
 
-**Note:** This API is disabled when MSC3861 is enabled. [See #15582](https://github.com/matrix-org/synapse/pull/15582)
+**Note:** This API is disabled when Matrix Authentication Service integration is enabled. Use the [MAS Admin API](https://element-hq.github.io/matrix-authentication-service/topics/admin-api.html) or [the MAS CLI](https://element-hq.github.io/matrix-authentication-service/reference/cli/manage.html#manage-promote-admin) instead. An easy way to make use of this functionality is provided by [Element Admin](https://element.io/en/server-suite/admin) as part of [ESS](https://element.io/en/server-suite) Community and Pro.
 
 Note that you cannot demote yourself.
 
@@ -910,7 +910,7 @@ delete largest/smallest or newest/oldest files first.
 
 ## Login as a user
 
-**Note:** This API is disabled when MSC3861 is enabled. [See #15582](https://github.com/matrix-org/synapse/pull/15582)
+**Note:** This API is disabled when Matrix Authentication Service integration is enabled. Use [Personal sessions](https://element-hq.github.io/matrix-authentication-service/topics/authorization.html#personal-sessions-personal-access-tokens) through the [MAS Admin API](https://element-hq.github.io/matrix-authentication-service/topics/admin-api.html) instead. An easy way to make use of this functionality is provided by [Element Admin](https://element.io/en/server-suite/admin) as part of [ESS](https://element.io/en/server-suite) Community and Pro.
 
 Get an access token that can be used to authenticate as that user. Useful for
 when admins wish to do actions on behalf of a user.