Drop support for Swift 5.2 and 5.3

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.

In this commit we drop support for Swift 5.2 and 5.3. We 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
+