Update supported kubernetes versions in version matrix (zalando#2402)

In v1.10.0 we we have started using policy API v1 for PDBs and policy API v1  is available from Kubernetes version 1.21. So, updating the kubernetes supported  version in matrix chart of v1.9, v1.10 to k8s 1.21+

Author: pavanreddymaley

README.md

@@ -57,8 +57,8 @@ production for over five years.
 
 | Release   | Postgres versions | K8s versions      | Golang  |
 | :-------- | :---------------: | :---------------: | :-----: |
-| v1.10.*   | 10 → 15      | 1.25+             | 1.19.8  |
-| v1.9.0    | 10 → 15      | 1.25+             | 1.18.9  |
+| v1.10.*   | 10 → 15      | 1.21+             | 1.19.8  |
+| v1.9.0    | 10 → 15      | 1.21+             | 1.18.9  |
 | v1.8.*    | 9.5 → 14     | 1.20 → 1.24  | 1.17.4  |
 | v1.7.1    | 9.5 → 14     | 1.20 → 1.24  | 1.16.9  |