Disable ExistentialAny temporarily (#10)

Disable ExistentialAny temporarily

### Motivation

Unfortunately until we adopt 5.9, adding ExistentialAny on upstream packages has unintended consequences for some downstream packages, so disabling for now. Details in apple/swift-openapi-generator#120

### Modifications

Disabled the feature enforcement, but the code changes are there, so downstream adopters can still use them.

### Result

We won't be seeing the issue described in apple/swift-openapi-generator#120.

### Test Plan

PR CI, which discovered the original issue.


Reviewed by: simonjbeaumont

Builds:
     ✔︎ pull request validation (5.8) - Build finished. 
     ✔︎ pull request validation (5.9) - Build finished. 
     ✔︎ pull request validation (nightly) - Build finished. 
     ✔︎ pull request validation (soundness) - Build finished. 

#10

Package.swift

@@ -16,11 +16,15 @@ import Foundation
 import PackageDescription
 
 // General Swift-settings for all targets.
-let swiftSettings: [SwiftSetting] = [
+var swiftSettings: [SwiftSetting] = []
+
+#if swift(>=5.9)
+swiftSettings.append(
     // 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-urlsession",