Move to Swift 5.9 as the minimum version (#80)

### Motivation

Part of addressing
apple/swift-openapi-generator#75 and
apple/swift-openapi-generator#119.

### Modifications

Bumped Swift tools version to 5.9 and made the `ExistentialAny` build
setting unconditional.

### Result

Building the package requires 5.9 now.

### Test Plan

Ran tests, all passed when using a Swift 5.9 toolchain.

Author: czechboy0

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version: 5.8
+// swift-tools-version: 5.9
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the SwiftOpenAPIGenerator open source project
@@ -15,15 +15,11 @@
 import PackageDescription
 
 // General Swift-settings for all targets.
-var swiftSettings: [SwiftSetting] = []
-
-#if swift(>=5.9)
-swiftSettings.append(
+let swiftSettings: [SwiftSetting] = [
     // https://github.com/apple/swift-evolution/blob/main/proposals/0335-existential-any.md
     // Require `any` for existential types.
     .enableUpcomingFeature("ExistentialAny")
-)
-#endif
+]
 
 let package = Package(
     name: "swift-openapi-runtime",

docker/Dockerfile

@@ -1,4 +1,4 @@
-ARG swift_version=5.8
+ARG swift_version=5.9
 ARG ubuntu_version=jammy
 ARG base_image=swift:${swift_version}-${ubuntu_version}
 

docker/docker-compose.2204.58.yaml

@@ -4,7 +4,7 @@
 #
 # % docker-compose \
 #   -f docker/docker-compose.yaml \
-#   -f docker/docker-compose.2204.58.yaml \
+#   -f docker/docker-compose.2204.59.yaml \
 #   run test
 #
 version: "3"