Replace deprecated StandardCharsets.UTF_8

The java.nio.charset.StandardCharsets.UTF_8 static field has been
deprecated. Replace with Charset.forName("UTF-8").

tests/org/elixir_lang/beam/DecompilerTest.java

@@ -9,8 +9,7 @@
 
 import java.io.File;
 import java.io.IOException;
-
-import static java.nio.charset.StandardCharsets.UTF_8;
+import java.nio.charset.Charset;
 
 public class DecompilerTest extends PlatformTestCase {
     /*
@@ -514,7 +513,7 @@ private void assertDecompiled(String name) throws IOException {
         String prefix = testDataPath + "/" + name + ".";
 
         File expectedFile = new File(prefix + "ex");
-        String expected = Files.toString(expectedFile, UTF_8);
+        String expected = Files.toString(expectedFile, Charset.forName("UTF-8"));
 
         VfsRootAccess.allowRootAccess(getTestRootDisposable(), testDataPath);