Raise minimum Swift version to 5.8 (#37)

Author: glbrntt

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.6
+// swift-tools-version:5.8
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the SwiftNIO open source project

docker/docker-compose.2004.56.yaml

@@ -6,8 +6,8 @@ services:
     image: swift-http-structured-headers:22.04-5.10
     build:
       args:
-        base_image: "swiftlang/swift:nightly-5.10-jammy"
         ubuntu_version: "jammy"
+        swift_version: "5.10"
 
   unit-tests:
     image: swift-http-structured-headers:22.04-5.10