Drop support for Swift 5.2 and 5.3 (#19)

As outlined in a [Swift forums post in November ’21](https://forums.swift.org/t/swiftnio-swift-version-support/53232), SwiftNIO will only support the latest non-patch Swift release and the 2 immediately prior non-patch versions.

- drop support for Swift 5.2 and 5.3. 
- update CI for Swift 5.4 to run on bionic instead of focal to ensure that we still test bionic.

Author: fabianfett

.swiftformat

@@ -1,6 +1,6 @@
 # file options
 
---swiftversion 5.2
+--swiftversion 5.4
 --exclude .build
 
 # format options

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.2
+// swift-tools-version:5.4
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the SwiftNIO open source project

docker/Dockerfile

@@ -1,4 +1,4 @@
-ARG swift_version=5.2
+ARG swift_version=5.4
 ARG ubuntu_version=bionic
 ARG base_image=swift:$swift_version-$ubuntu_version
 FROM $base_image

docker/docker-compose.1604.52.yaml

@@ -0,0 +1,20 @@
+version: "3"
+
+services:
+
+  runtime-setup:
+    image: swift-http-structured-headers:18.04-5.4
+    build:
+      args:
+        ubuntu_version: "bionic"
+        swift_version: "5.4"
+
+  unit-tests:
+    image: swift-http-structured-headers:18.04-5.4
+
+  test:
+    image: swift-http-structured-headers:18.04-5.4
+
+  shell:
+    image: swift-http-structured-headers:18.04-5.4
+