Gate ExperimentalCustomExecutors usage behind Swift 6.4 compiler check

Seems like it's dropped in Swift 6.3 rc shipped along with Xcode 26.4 RC
in the last minute

Author: kateinoigakukun

Sources/JavaScriptEventLoop/JavaScriptEventLoop+ExecutorFactory.swift

@@ -4,14 +4,14 @@
 // See: https://github.com/swiftlang/swift/pull/80266
 // See: https://forums.swift.org/t/pitch-2-custom-main-and-global-executors/78437
 
-#if compiler(>=6.3)
+#if compiler(>=6.4)
 @_spi(ExperimentalCustomExecutors) import _Concurrency
 #else
 import _Concurrency
 #endif
 import _CJavaScriptKit
 
-#if compiler(>=6.3)
+#if compiler(>=6.4)
 
 // MARK: - MainExecutor Implementation
 // MainExecutor is used by the main actor to execute tasks on the main thread
@@ -111,4 +111,4 @@ extension JavaScriptEventLoop: ExecutorFactory {
     }
 }
 
-#endif  // compiler(>=6.3)
+#endif  // compiler(>=6.4)

Sources/JavaScriptEventLoop/JavaScriptEventLoop.swift

@@ -1,5 +1,5 @@
 import JavaScriptKit
-#if compiler(>=6.3)
+#if compiler(>=6.4)
 @_spi(ExperimentalCustomExecutors) import _Concurrency
 #else
 import _Concurrency
@@ -123,9 +123,9 @@ public final class JavaScriptEventLoop: SerialExecutor, @unchecked Sendable {
     private static func installGlobalExecutorIsolated() {
         guard !didInstallGlobalExecutor else { return }
         didInstallGlobalExecutor = true
-        #if compiler(>=6.3)
+        #if compiler(>=6.4)
         if #available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, visionOS 9999, *) {
-            // For Swift 6.3 and above, we can use the new `ExecutorFactory` API
+            // For Swift 6.4 and above, we can use the new `ExecutorFactory` API
             _Concurrency._createExecutors(factory: JavaScriptEventLoop.self)
         }
         #else