Update changelog from #11867 to be a single line.

changelog.d/11867.feature

-Stabilize [MSC3231](https://github.com/matrix-org/matrix-doc/pull/3231).
-
-Client implementations using `m.login.registration_token` should switch to the stable identifiers:
-* `org.matrix.msc3231.login.registration_token` in query parameters and request/response bodies becomes `m.login.registration_token`.
-* `/_matrix/client/unstable/org.matrix.msc3231/register/org.matrix.msc3231.login.registration_token/validity` becomes `/_matrix/client/v1/register/m.login.registration_token/validity`.
\ No newline at end of file
+Stabilize support for [MSC3231](https://github.com/matrix-org/matrix-doc/pull/3231). Clients should switch to the stable identifier and endpoint.