remove mention of gitter

Author: baronfel

src/System.CommandLine/README.md

@@ -277,5 +277,4 @@ We welcome contributions! Please see our [Contributing Guide](https://github.com
 ## Support
 
 - **Issues**: [GitHub Issues](https://github.com/dotnet/command-line-api/issues)
-- **Discussions**: [GitHub Discussions](https://github.com/dotnet/command-line-api/discussions)
-- **Chat**: [Gitter Community](https://gitter.im/dotnet/command-line-api)
+- **Discussions**: [GitHub Discussions](https://github.com/dotnet/command-line-api/discussions)