Raise minimum supported Swift version from 5.4 to 5.5 (#27)

Motivation:

We periodically drop support for older Swift versions. Now that
5.7 has been released, 5.4 can be dropped.

Modifications:

- Remove 5.4 specific Package.swift and docker-compose
- Update the 5.7 docker-compose to use the released 5.7 and move from
  focal (2004) to jammy (2204)

Results:

Minimum Swift version is 5.5

Author: glbrntt

Package@swift-5.4.swift

@@ -43,7 +43,7 @@ let field = Array("Sec-CH-Example, Sec-CH-Example-2".utf8)
 
 struct AcceptCH: StructuredFieldValue {
     static let structuredFieldType: StructuredFieldType = .list
-    
+
     var items: [String]
 }
 

README.md

@@ -1,5 +1,5 @@
-ARG swift_version=5.4
-ARG ubuntu_version=bionic
+ARG swift_version=5.7
+ARG ubuntu_version=focal
 ARG base_image=swift:$swift_version-$ubuntu_version
 FROM $base_image
 # needed to do again after FROM due to docker limitation

docker/Dockerfile

@@ -0,0 +1,20 @@
+version: "3"
+
+services:
+
+  runtime-setup:
+    image: swift-http-structured-headers:22.04-5.7
+    build:
+      args:
+        ubuntu_version: "jammy"
+        swift_version: "5.7"
+
+  unit-tests:
+    image: swift-http-structured-headers:22.04-5.7
+
+  test:
+    image: swift-http-structured-headers:22.04-5.7
+
+  shell:
+    image: swift-http-structured-headers:22.04-5.7
+