Set Swift tools version to 6.0

This is required because the new Swift Testing framework is included in the Swift 6 toolchain and Xcode 16.

Author: mbernson

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version: 5.9
+// swift-tools-version: 6.0
 
 import PackageDescription
 

Sources/SwiftGit2/CheckoutStrategy.swift

@@ -10,7 +10,7 @@ import Clibgit2
 
 /// The flags defining how a checkout should be performed.
 /// More detail is available in the libgit2 documentation for `git_checkout_strategy_t`.
-public struct CheckoutStrategy: OptionSet {
+public struct CheckoutStrategy: OptionSet, Sendable {
 	private let value: UInt
 
 	// MARK: - Initialization

Sources/SwiftGit2/Diffs.swift

@@ -60,7 +60,7 @@ public struct Diff {
 		}
 	}
 
-	public struct Status: OptionSet {
+	public struct Status: OptionSet, Sendable {
 		// This appears to be necessary due to bug in Swift
 		// https://bugs.swift.org/browse/SR-3003
 		public init(rawValue: UInt32) {
@@ -84,7 +84,7 @@ public struct Diff {
 		public static let conflicted             = Status(rawValue: GIT_STATUS_CONFLICTED.rawValue)
 	}
 
-	public struct Flags: OptionSet {
+	public struct Flags: OptionSet, Sendable {
 		// This appears to be necessary due to bug in Swift
 		// https://bugs.swift.org/browse/SR-3003
 		public init(rawValue: UInt32) {

Sources/SwiftGit2/StatusOptions.swift

@@ -5,7 +5,7 @@
 import Clibgit2
 import Foundation
 
-public struct StatusOptions: OptionSet {
+public struct StatusOptions: OptionSet, Sendable {
 	public let rawValue: UInt32
 
 	public init(rawValue: UInt32) {