Drop support for Swift 5.2 and 5.3 (#337)

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

@@ -24,7 +24,7 @@ To depend on `swift-nio-http2`, put the following in the `dependencies` of your
 
     .package(url: "https://github.com/apple/swift-nio-http2.git", from: "1.19.2"),
 
-SwiftNIO HTTP2 1.18.x and later support Swift 5.2 and above. 1.17.x and earlier also support Swift 5.0 and 5.1.
+SwiftNIO HTTP2 1.21.x and later support Swift 5.4 and above. 1.20.x and earlier also support Swift 5.2 and 5.3. 1.17.x and earlier also support Swift 5.0 and 5.1.
 
 ### `swift-nio-http2` 0.x
 

docker/docker-compose.2004.54.yaml → docker/docker-compose.1804.54.yaml

@@ -3,7 +3,7 @@ version: "3"
 services:
 
   runtime-setup:
-    image: swift-nio-http2:20.04-5.4
+    image: swift-nio-http2:18.04-5.4
     build:
       args:
         base_image: "swift:5.4-focal"
@@ -12,19 +12,19 @@ services:
         h2spec_version: "2.2.1"
 
   unit-tests:
-    image: swift-nio-http2:20.04-5.4
+    image: swift-nio-http2:18.04-5.4
 
   integration-tests:
-    image: swift-nio-http2:20.04-5.4
+    image: swift-nio-http2:18.04-5.4
 
   performance-test:
-    image: swift-nio-http2:20.04-5.4
+    image: swift-nio-http2:18.04-5.4
 
   h2spec:
-    image: swift-nio-http2:20.04-5.4
+    image: swift-nio-http2:18.04-5.4
 
   test:
-    image: swift-nio-http2:20.04-5.4
+    image: swift-nio-http2:18.04-5.4
     environment:
       - MAX_ALLOCS_ALLOWED_1k_requests_interleaved=46200
       - MAX_ALLOCS_ALLOWED_1k_requests_noninterleaved=45100
@@ -39,4 +39,4 @@ services:
       - MAX_ALLOCS_ALLOWED_stream_teardown_100_concurrent=303150
 
   shell:
-    image: swift-nio-http2:20.04-5.4
+    image: swift-nio-http2:18.04-5.4

scripts/build_podspec.sh

@@ -102,7 +102,7 @@ Pod::Spec.new do |s|
   s.documentation_url = 'https://apple.github.io/swift-nio-http2/'
   s.module_name = '${target#Swift}'
 
-  s.swift_version = '5.2'
+  s.swift_version = '5.4'
   s.cocoapods_version = '>=1.6.0'
   s.ios.deployment_target = '10.0'
   s.osx.deployment_target = '10.12'

scripts/check_no_api_breakages.sh

@@ -18,7 +18,7 @@ set -eu
 function usage() {
     echo >&2 "Usage: $0 REPO-GITHUB-URL NEW-VERSION OLD-VERSIONS..."
     echo >&2
-    echo >&2 "This script requires a Swift 5.2+ toolchain."
+    echo >&2 "This script requires a Swift 5.6+ toolchain."
     echo >&2
     echo >&2 "Examples:"
     echo >&2