add the unsupported note to require_secure_transport (#15034) (#15086)

system-variables.md

@@ -718,10 +718,14 @@ mysql> SHOW GLOBAL VARIABLES LIKE 'max_prepared_stmt_count';
 
 ### require_secure_transport <span class="version-mark">New in v6.1.0</span>
 
+> **Note:**
+>
+> Currently, this variable is not supported on [TiDB Dedicated](https://docs.pingcap.com/tidbcloud/select-cluster-tier#tidb-dedicated). DO **NOT** enable this variable for TiDB Dedicated clusters. Otherwise, you might get SQL client connection failures. This restriction is a temporary control measure and will be resolved in a future release.
+
 - Scope: GLOBAL
 - Persists to cluster: Yes
 - Type: Boolean
-- Default value: `OFF`
+- Default value: `OFF` for TiDB Self-Hosted and [TiDB Dedicated](https://docs.pingcap.com/tidbcloud/select-cluster-tier#tidb-dedicated), `ON` for [TiDB Serverless](https://docs.pingcap.com/tidbcloud/select-cluster-tier#tidb-serverless)
 
 <CustomContent platform="tidb">