Drop support for Swift 5.8 (#259)

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.8
+// swift-tools-version:5.9
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the SwiftCrypto open source project

README.md

@@ -117,7 +117,8 @@ Swift Crypto      | Minimum Swift Version
 `2.2.0 ..< 2.4.2` | 5.5
 `2.4.2 ..< 3.1.0` | 5.6
 `3.1.0 ..< 3.3.0` | 5.7
-`3.3.0 ...`       | 5.8
+`3.3.0 ..< 3.8.0` | 5.8
+`3.8.0 ...`       | 5.9
 
 ### Compatibility
 

Sources/_CryptoExtras/AES/AES_GCM_SIV.swift

@@ -16,11 +16,7 @@ import Crypto
 @_implementationOnly import CCryptoBoringSSL
 @_implementationOnly import CCryptoBoringSSLShims
 @_implementationOnly import CryptoBoringWrapper
-#if canImport(Darwin) || swift(>=5.9.1)
 import Foundation
-#else
-@preconcurrency import Foundation
-#endif
 
 /// Types associated with the AES GCM SIV algorithm
 extension AES.GCM {

Sources/_CryptoExtras/ChaCha20CTR/ChaCha20CTR.swift

@@ -16,11 +16,7 @@
 @_implementationOnly import CCryptoBoringSSLShims
 import Crypto
 @_implementationOnly import CryptoBoringWrapper
-#if canImport(Darwin) || swift(>=5.9.1)
 import Foundation
-#else
-@preconcurrency import Foundation
-#endif
 
 typealias ChaCha20CTRImpl = OpenSSLChaCha20CTRImpl
 

Sources/_CryptoExtras/RSA/RSA.swift

@@ -31,12 +31,8 @@ fileprivate typealias BackingPrivateKey = BoringSSLRSAPrivateKey
 ///
 /// When rolling out new cryptosystems, users should avoid RSA and use ECDSA or edDSA instead. RSA
 /// support is provided for interoperability with legacy systems.
-#if swift(>=5.8)
 @_documentation(visibility: public)
 public enum _RSA { }
-#else
-public enum _RSA { }
-#endif
 
 extension _RSA {
     public enum Signing { }