Require only JNI version 1.6

Based on our uses of the JNI APIs, the JavaKit path (Swift wrapping
Java classes) only requires JNI as of Java 1.6, far earlier than the
JDK 21 we had previously required.

Author: DougGregor

Sources/JavaKitVM/JavaVirtualMachine.swift

@@ -41,7 +41,7 @@ public final class JavaVirtualMachine: @unchecked Sendable {
     var jvm: JavaVMPointer? = nil
     var environment: UnsafeMutableRawPointer? = nil
     var vmArgs = JavaVMInitArgs()
-    vmArgs.version = JNI_VERSION_21
+    vmArgs.version = JNI_VERSION_1_6
     vmArgs.ignoreUnrecognized = jboolean(ignoreUnrecognized ? JNI_TRUE : JNI_FALSE)
 
     // Construct the complete list of VM options.