Use try-with-resources where possible (#6765)

core/src/main/java/hudson/Util.java

@@ -625,7 +625,7 @@ public class Util {
      */
     @NonNull
     public static String getDigestOf(@NonNull InputStream source) throws IOException {
-        try {
+        try (source) {
             MessageDigest md5 = getMd5();
             DigestInputStream in = new DigestInputStream(source, md5);
             // Note: IOUtils.copy() buffers the input internally, so there is no
@@ -634,8 +634,6 @@ public class Util {
             return toHexString(md5.digest());
         } catch (NoSuchAlgorithmException e) {
             throw new IOException("MD5 not installed", e);    // impossible
-        } finally {
-            source.close();
         }
         /* JENKINS-18178: confuses Maven 2 runner
         try {