Update 3.10.0 changelog with idle_change format change

Create a section on the management plugin and mention it is a
potential breaking change for clients of the REST API.

Author: acogoluegnes

release-notes/3.10.0.md

@@ -209,6 +209,16 @@ This release includes all bug fixes that shipped in `3.9.x` releases.
 
    GitHub issue: [#3341](https://github.com/rabbitmq/rabbitmq-server/pull/3341)
 
+### Management Plugin
+
+ * `idle_since` field now uses RFC 3339 format.
+
+   ![Breaking change](https://img.shields.io/badge/-Breaking%20change-red) This is a **potentially breaking change**.
+   Sample value with the previous format: `2022-03-22 11:39:37`. With the new format: `2022-03-22T11:39:37.908+01:00`.
+   Clients of the REST API should be able to parse values in the new format.
+
+   GitHub issue: [#4276](https://github.com/rabbitmq/rabbitmq-server/issues/4276)
+
 
 ### OAuth 2 AuthN and AuthZ Backend Plugin