Drop 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.

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