Update swift tools version to 5.8 (#41)

* build: update swift tools version to 5.8

* docs: update readme to remove swift 5.6 badge

Author: GNMoseke

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.6
+// swift-tools-version:5.8
 import PackageDescription
 
 let package = Package(

README.md

@@ -18,9 +18,6 @@
     <a href="https://github.com/vapor/queues-redis-driver/actions/workflows/test.yml">
         <img src="https://github.com/vapor/queues-redis-driver/actions/workflows/test.yml/badge.svg?event=push" alt="Continuous Integration">
     </a>
-    <a href="https://swift.org">
-        <img src="http://img.shields.io/badge/swift-5.6-brightgreen.svg" alt="Swift 5.6">
-    </a>
     <a href="https://swift.org">
         <img src="http://img.shields.io/badge/swift-5.8-brightgreen.svg" alt="Swift 5.8">
     </a>