Add changelog

Related to https://github.com/matrix-org/synapse/issues/15089

Element Web PR to honor `org.matrix.msc3030.stable`: https://github.com/matrix-org/matrix-react-sdk/pull/10398

changelog.d/15283.feature

@@ -0,0 +1 @@
+Add stable unstable feature (`org.matrix.msc3030.stable`) for [MSC3030](https://github.com/matrix-org/matrix-spec-proposals/pull/3030) jump to date [before Matrix `v1.6` is fully supported on Synapse](https://github.com/matrix-org/synapse/issues/15089).

synapse/rest/client/versions.py

@@ -111,6 +111,10 @@ class VersionsRestServlet(RestServlet):
                     "fi.mau.msc2815": self.config.experimental.msc2815_enabled,
                     # Adds a ping endpoint for appservices to check HS->AS connection
                     "fi.mau.msc2659": self.config.experimental.msc2659_enabled,
+                    # Adds support for jump to date endpoints (/timestamp_to_event) as per MSC3030.
+                    # TODO: Remove when Synapse advertises support for `v1.6` which includes MSC3030
+                    # (https://github.com/matrix-org/synapse/issues/15089)
+                    "org.matrix.msc3030.stable": True,
                     # Adds support for login token requests as per MSC3882
                     "org.matrix.msc3882": self.config.experimental.msc3882_enabled,
                     # Adds support for remotely enabling/disabling pushers, as per MSC3881