Update statement about support grafana versions

https://grafana.com/developers/plugin-tools#supported-grafana-version

> We generally recommend that you build for a version of Grafana later than v9.0.

README.md

@@ -1,6 +1,6 @@
 # Checkmk data source
 
-[![Grafana 8.0](https://img.shields.io/badge/Grafana-8.0-orange)](https://www.grafana.com)
+[![Grafana 9.0](https://img.shields.io/badge/Grafana-9.0-orange)](https://www.grafana.com)
 [![Checkmk cloud data source](https://img.shields.io/badge/dynamic/json?color=blue&label=Checkmk%20for%20Cloud%20Edition&query=%24.version&url=https%3A%2F%2Fgrafana.com%2Fapi%2Fplugins%2Fcheckmk-cloud-datasource)](https://grafana.com/grafana/plugins/checkmk-cloud-datasource)
 [![Checkmk unsigned data source](https://img.shields.io/badge/dynamic/json?color=blue&label=Checkmk&query=tag_name&url=https%3A%2F%2Fapi.github.com%2Frepos%2Ftribe29%2Fgrafana-checkmk-datasource%2Freleases%2Flatest)](https://github.com/Checkmk/grafana-checkmk-datasource)
 [![CI](https://github.com/Checkmk/grafana-checkmk-datasource/actions/workflows/ci.yml/badge.svg)](https://github.com/Checkmk/grafana-checkmk-datasource/actions/workflows/ci.yml?query=event%3Aschedule)
@@ -15,7 +15,7 @@ This [data source][2] plugin for [Grafana][1] allows to address Checkmk as sourc
 
 To make use of the plugin, you need to take care the correct versions are installed. You need to match both, the Grafana and the Checkmk version:
 
-- **Grafana 8.0.0 or higher**
+- **Grafana 9.0.0 or higher**
 - **Checkmk Cloud Edition 2.2.0 or higher** for the signed plugin available from [Grafana][6]
 - **Checkmk 2.1.0 or higher** for the unsigned plugin available from [Github][8]