Drop Swift 5.2 and 5.3 (#262)

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.

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
 
 import PackageDescription
 

docker/Dockerfile

@@ -1,5 +1,4 @@
-ARG swift_version=5.2
-# FIXME when 5.2 images are available
+ARG swift_version=5.4
 ARG base_image=swift:$swift_version-amazonlinux2
 FROM $base_image
 # needed to do again after FROM due to docker limitation

docker/Dockerfile.ubuntu

@@ -17,7 +17,7 @@ If you have never used AWS Lambda or Docker before, check out this [getting star
 First, create a SwiftPM project and pull Swift AWS Lambda Runtime as dependency into your project
 
  ```swift
- // swift-tools-version:5.2
+ // swift-tools-version:5.6
 
  import PackageDescription
 
@@ -80,7 +80,7 @@ Next, create a `main.swift` and implement your Lambda.
 First, add a dependency on the event packages:
 
  ```swift
- // swift-tools-version:5.2
+ // swift-tools-version:5.6
 
  import PackageDescription