book: set v1.1.x EOL date

Signed-off-by: Stefan Büringer [email protected]

Author: sbueringer

CONTRIBUTING.md

@@ -113,7 +113,8 @@ Cluster API maintains the most recent release branch for all supported API and c
 
 | API Version   | Branch      | Supported Until |
 | ------------- |-------------|-----------------|
-| **v1beta1**   | release-1.1 | current stable  |
+| **v1beta1**   | release-1.2 | current stable  |
+| **v1beta1**   | release-1.1 | 2022-09-15      |
 | **v1beta1**   | release-1.0 | 2022-02-02      |
 | **v1alpha4**  | release-0.4 | 2022-04-06      |
 | **v1alpha3**  | release-0.3 | 2022-02-23      |