Drop support for Swift 5.2 and 5.3 (#154)

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.

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

README.md

@@ -14,7 +14,7 @@ All code will go through code review like in the other repositories related to t
 `swift-nio-extras` part of the SwiftNIO 2 family of repositories and depends on the following:
 
 - [`swift-nio`](https://github.com/apple/swift-nio), version 2.30.0 or better.
-- Swift 5.2.
+- Swift 5.4.
 - `zlib` and its development headers installed on the system. But don't worry, you'll find `zlib` on pretty much any UNIX system that can compile any sort of code.
 
 To depend on `swift-nio-extras`, put the following in the `dependencies` of your `Package.swift`:
@@ -25,7 +25,7 @@ To depend on `swift-nio-extras`, put the following in the `dependencies` of your
 
 ### Support for older Swift versions
 
-Earlier versions of SwiftNIO (2.29.x and lower) and SwiftNIOExtras (1.9.x and lower) supported Swift 5.0 and 5.1. 
+Earlier versions of SwiftNIO (2.39.x and lower) and SwiftNIOExtras (1.10.x and lower) supported Swift 5.2 and 5.3, SwiftNIO (2.29.x and lower) and SwiftNIOExtras (1.9.x and lower) supported Swift 5.0 and 5.1. 
 
 On the [`nio-extras-0.1`](https://github.com/apple/swift-nio-extras/tree/nio-extras-0.1) branch, you can find the `swift-nio-extras` version for the SwiftNIO 1 family. It requires Swift 4.1 or better.
 

docker/Dockerfile

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

docker/docker-compose.1804.54.yaml

@@ -0,0 +1,17 @@
+version: "3"
+
+services:
+
+  runtime-setup:
+    image: swift-nio-extras:18.04-5.4
+    build:
+      args:
+        base_image: "swift:5.4-bionic"
+        ubuntu_version: "bionic"
+        swift_version: "5.4"
+
+  test:
+    image: swift-nio-extras:18.04-5.4
+
+  shell:
+    image: swift-nio-extras:18.04-5.4