Update README.md, stac-fastapi badge to 6.0.0

Author: jonhealy1

README.md

@@ -15,7 +15,7 @@
   [![GitHub forks](https://img.shields.io/github/forks/stac-utils/stac-fastapi-elasticsearch-opensearch.svg?color=blue)](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/network/members)
    [![PyPI version](https://img.shields.io/pypi/v/stac-fastapi-elasticsearch.svg?color=blue)](https://pypi.org/project/stac-fastapi-elasticsearch/)
   [![STAC](https://img.shields.io/badge/STAC-1.1.0-blue.svg)](https://github.com/radiantearth/stac-spec/tree/v1.1.0)
-  [![stac-fastapi](https://img.shields.io/badge/stac--fastapi-5.2.0-blue.svg)](https://github.com/stac-utils/stac-fastapi)
+  [![stac-fastapi](https://img.shields.io/badge/stac--fastapi-6.0.0-blue.svg)](https://github.com/stac-utils/stac-fastapi)
 
 ## Sponsors & Supporters