fix: 修复 Java 8 兼容性问题

- Files.writeString() -> Files.write(path, bytes)
- Files.readString() -> new String(Files.readAllBytes(path))
- Path.of() -> Paths.get()

Author: creeperCN

src/main/java/com/github/balloonupdate/mcpatch/client/selfupdate/SelfUpdateDownloader.java

@@ -163,7 +163,7 @@ public static void createUpdateMarker(Path newJarPath) throws Exception {
         content.append("path=").append(newJarPath.toAbsolutePath()).append("\n");
         content.append("time=").append(System.currentTimeMillis()).append("\n");
 
-        Files.writeString(markerFile, content.toString());
+        Files.write(markerFile, content.toString().getBytes());
 
         Log.debug("已创建更新标记文件: " + markerFile);
     }

src/main/java/com/github/balloonupdate/mcpatch/client/selfupdate/SelfUpdateInstaller.java

@@ -7,6 +7,7 @@
 import java.io.StringReader;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.nio.file.Paths;
 import java.nio.file.StandardCopyOption;
 
 /**
@@ -34,7 +35,7 @@ public static boolean installPendingUpdate() {
             Log.debug("读取标记文件: " + markerFile);
 
             // 解析标记文件
-            String markerContent = Files.readString(markerFile);
+            String markerContent = new String(Files.readAllBytes(markerFile));
             Path newJarPath = parseMarkerFile(markerContent);
 
             if (newJarPath == null) {
@@ -113,15 +114,15 @@ private static Path parseMarkerFile(String content) {
 
             while ((line = reader.readLine()) != null) {
                 if (line.startsWith("path=")) {
-                    jarPath = Path.of(line.substring(5).trim());
+                    jarPath = Paths.get(line.substring(5).trim());
                 }
             }
 
             // 如果没有 path= 格式，尝试直接作为路径解析（旧格式兼容）
             if (jarPath == null && !content.trim().isEmpty()) {
                 String firstLine = content.split("\n")[0].trim();
                 if (!firstLine.isEmpty()) {
-                    jarPath = Path.of(firstLine);
+                    jarPath = Paths.get(firstLine);
                 }
             }