Raise minimum supported Swift version from 5.4 to 5.5 (#131)

Motivation:

Old Swift versions are periodically dropped. Now that 5.7 has been
released, 5.4 will be dropped.

Modifications:

- Update tools version
- Remove 5.4 docker-compose
- Update 5.7 docker-compose to use released image and move from focal to
  jammy
- Update docs

Result:

Swift 5.5 is the minimum supported Swift version

Author: glbrntt

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.4
+// swift-tools-version:5.5
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the SwiftCrypto open source project

README.md

@@ -108,7 +108,13 @@ If you believe you have identified a vulnerability in Swift Crypto, please [repo
 
 ### Swift versions
 
-Swift Crypto supports Swift 5.4 and later. Swift Crypto supported Swift 5.2 and 5.3 until version 2.1 and Swift 5.1 until version 2.0.
+The most recent versions of Swift Crypto support Swift 5.5 and newer. The minimum Swift version supported by Swift Crypto releases are detailed below:
+
+Swift Crypto      | Minimum Swift Version
+------------------|----------------------
+`2.0.0 ..< 2.1.0` | 5.2
+`2.1.0 ..< 2.2.0` | 5.4
+`2.2.0 ...`       | 5.5
 
 ### Compatibility
 

docker/Dockerfile

@@ -1,5 +1,5 @@
-ARG swift_version=5.0
-ARG ubuntu_version=bionic
+ARG swift_version=5.7
+ARG ubuntu_version=jammy
 ARG base_image=swift:$swift_version-$ubuntu_version
 FROM $base_image
 # needed to do again after FROM due to docker limitation

docker/docker-compose.1804.54.yaml

@@ -0,0 +1,21 @@
+version: "3"
+
+services:
+
+  runtime-setup:
+    image: swift-crypto:22.04-5.7
+    build:
+      args:
+        ubuntu_version: "jammy"
+        swift_version: "5.7"
+
+  test:
+    image: swift-crypto:22.04-5.7
+    environment: []
+      #- SANITIZER_ARG=--sanitize=thread
+
+  cmake:
+    image: swift-crypto:22.04-5.7
+
+  shell:
+    image: swift-crypto:22.04-5.7