Set supported swift versions to 5.6 - 5.8

Author: adam-fowler

README.md

@@ -1,7 +1,7 @@
 # MQTT NIO
 
 [![sswg:sandbox|94x20](https://img.shields.io/badge/sswg-sandbox-lightgrey.svg)](https://github.com/swift-server/sswg/blob/master/process/incubation.md#sandbox-level)
-[<img src="http://img.shields.io/badge/swift-5.7-brightgreen.svg" alt="Swift 5.7" />](https://swift.org)
+[<img src="https://img.shields.io/badge/swift-5.6_to_5.8-brightgreen.svg" alt="Swift 5.8" />](https://swift.org)
 [<img src="https://github.com/adam-fowler/mqtt-nio/workflows/CI/badge.svg" />](https://github.com/adam-fowler/mqtt-nio/workflows/CI/badge.svg)
 
 A Swift NIO based MQTT v3.1.1 and v5.0 client.