Fix MalformedInputException on Windows with Java 8 due to charset mismatch in stale data cache

Changed StaleHelper to always save in UTF-8 instead of
platform-dependent default charset. This ensures consistency with
AbstractJavadocMojo.isUpToDate() which reads the stale data file using UTF-8.

Previously on Windows with Java 8:
- First run: file written with Cp1252 (default charset)
- Second run: file read with UTF-8, causing MalformedInputException

Author: slachiewicz

src/main/java/org/apache/maven/plugins/javadoc/StaleHelper.java

@@ -20,8 +20,6 @@
 
 import java.io.File;
 import java.io.IOException;
-import java.nio.charset.Charset;
-import java.nio.charset.StandardCharsets;
 import java.nio.file.DirectoryStream;
 import java.nio.file.Files;
 import java.nio.file.Path;
@@ -31,29 +29,15 @@
 import java.util.List;
 
 import org.apache.maven.reporting.MavenReportException;
-import org.codehaus.plexus.languages.java.version.JavaVersion;
 import org.codehaus.plexus.util.cli.Commandline;
 
+import static java.nio.charset.StandardCharsets.UTF_8;
+
 /**
  * Helper class to compute and write data used to detect a
  * stale javadoc.
  */
 public class StaleHelper {
-
-    /**
-     * Compute the encoding of the stale javadoc
-     *
-     * @return the encoding of the stale data
-     */
-    private static Charset getDataCharset() {
-        if (JavaVersion.JAVA_SPECIFICATION_VERSION.isAtLeast("9")
-                && JavaVersion.JAVA_SPECIFICATION_VERSION.isBefore("12")) {
-            return StandardCharsets.UTF_8;
-        } else {
-            return Charset.defaultCharset();
-        }
-    }
-
     /**
      * Compute the data used to detect a stale javadoc
      *
@@ -72,7 +56,7 @@ public static List<String> getStaleData(Commandline cmd) throws MavenReportExcep
             for (String arg : args) {
                 if (arg.startsWith("@")) {
                     String name = arg.substring(1);
-                    options.addAll(Files.readAllLines(dir.resolve(name), getDataCharset()));
+                    options.addAll(Files.readAllLines(dir.resolve(name), UTF_8));
                     ignored.add(name);
                 }
             }
@@ -123,7 +107,7 @@ public static void writeStaleData(Commandline cmd, Path path) throws MavenReport
         try {
             List<String> curdata = getStaleData(cmd);
             Files.createDirectories(path.getParent());
-            Files.write(path, curdata, getDataCharset());
+            Files.write(path, curdata, UTF_8);
         } catch (IOException e) {
             throw new MavenReportException("Error checking stale data", e);
         }