Update Kubernetes support matrix

[FeynmanZhou]

Kubernetes follows an N-2 support policy (meaning that the 3 most recent minor versions receive security and bug fixes) along with a 15-week release cycle. At the end of the day, the most recent three minor Kubernetes releases are 1.32, 1.31, 1.30, which are officially supported by the Kubernetes community.

Looking at the Kubernetes test matrix, Kubernetes v1.29 and v1.30 are covered now. We need to bump up the Kubernetes versions to 1.30 and 1.31 respectively since v1.29 is no longer maintained by Kubernetes community.

In addition, the prerequisite of Ratify installation just mention Kubernetes v1.20 or higher on the website. This is not a strict prerequisite for users. I would suggest adding a supported Kubernetes matrix statement to the installation guide and included the recommended versions (tested by Ratify e2e) there.

[FeynmanZhou]

cc @binbin-li