README: Update badge to signal compatibility with Grafana 12

`grafana-wtf` includes `grafana-client`, and runs integration tests
against Grafana 12.

Author: amotl

README.md

@@ -5,7 +5,7 @@
 [![License](https://img.shields.io/github/license/grafana-toolbox/grafana-client.svg?style=flat-square)](https://github.com/grafana-toolbox/grafana-client/blob/main/LICENSE)
 
 [![Python versions](https://img.shields.io/pypi/pyversions/grafana-client.svg?style=flat-square)](https://pypi.org/project/grafana-client/)
-[![Grafana versions](https://img.shields.io/badge/Grafana-5.x%20--%2011.x-blue.svg?style=flat-square)](https://github.com/grafana/grafana)
+[![Grafana versions](https://img.shields.io/badge/Grafana-5.x%20--%2012.x-blue.svg?style=flat-square)](https://github.com/grafana/grafana)
 
 [![Status](https://img.shields.io/pypi/status/grafana-client.svg?style=flat-square)](https://pypi.org/project/grafana-client/)
 [![PyPI](https://img.shields.io/pypi/v/grafana-client.svg?style=flat-square)](https://pypi.org/project/grafana-client/)