Update testbed matrix for v1.12.1

Signed-off-by: danfengl <danfengl@vmware.com>

README.md

@@ -42,7 +42,7 @@ The following is a list of the supported Kubernetes versions for each Velero ver
 
 | Velero version | Expected Kubernetes version compatibility | Tested on Kubernetes version           |
 |----------------|-------------------------------------------|----------------------------------------|
-| 1.12           | 1.18-latest                               | 1.25.7, 1.26.5, 1.26.7, and 1.27.3     |
+| 1.12           | 1.18-latest                               | 1.25.7, 1.26.5, 1.27.3 and 1.28.0      |
 | 1.11           | 1.18-latest                               | 1.23.10, 1.24.9, 1.25.5, and 1.26.1    |
 | 1.10           | 1.18-latest                               | 1.22.5, 1.23.8, 1.24.6 and 1.25.1      |
 | 1.9            | 1.18-latest                               | 1.20.5, 1.21.2, 1.22.5, 1.23, and 1.24 |