[docs] Add supported Flink versions for Flink CDC 2.1(#611)

Author: mustang2247

docs/content/about.md

@@ -28,6 +28,7 @@ The version mapping between Flink CDC Connectors and Flink.
 |1.3.0 | 1.12.* |
 |1.4.0 | 1.13.* |
 |2.0.* | 1.13.* |
+|2.1.* | 1.13.* |
 
 ## Features