Change/remove usage of some deprecated JDK API

API deprecated in Java 16 or more. There are alternatives
(e.g. for certificate subject and issuer) for some, but not for
all (e.g. usage of the security manager to change thread, which
is no big deal as the changes to thread are minor and it is
likely nobody cares about such checks nowadays).

Fixes #709

(cherry picked from commit 57b61d8)

Author: acogoluegnes

src/main/java/com/rabbitmq/client/impl/Environment.java

@@ -23,32 +23,29 @@
  * Package-protected API.
  */
 public class Environment {
+
+    /**
+     * This method is deprecated and subject to removal in the next major release.
+     *
+     * There is no replacement for this method, as it used to use the
+     * {@link SecurityManager}, which is itself deprecated and subject to removal.
+     * @deprecated
+     * @return always returns true
+     */
+    @Deprecated
     public static boolean isAllowedToModifyThreads() {
-        try {
-            SecurityManager sm = System.getSecurityManager();
-            if(sm != null) {
-                sm.checkPermission(new RuntimePermission("modifyThread"));
-                sm.checkPermission(new RuntimePermission("modifyThreadGroup"));
-            }
-            return true;
-        } catch (SecurityException se) {
-            return false;
-        }
+       return true;
     }
 
     public static Thread newThread(ThreadFactory factory, Runnable runnable, String name) {
         Thread t = factory.newThread(runnable);
-        if(isAllowedToModifyThreads()) {
-            t.setName(name);
-        }
+        t.setName(name);
         return t;
     }
 
     public static Thread newThread(ThreadFactory factory, Runnable runnable, String name, boolean isDaemon) {
         Thread t = newThread(factory, runnable, name);
-        if(isAllowedToModifyThreads()) {
-            t.setDaemon(isDaemon);
-        }
+        t.setDaemon(isDaemon);
         return t;
     }
 }

src/main/java/com/rabbitmq/client/impl/TlsUtils.java

@@ -104,7 +104,7 @@ public static String peerCertificateInfo(Certificate certificate, String prefix)
         try {
             return String.format("%s subject: %s, subject alternative names: %s, " +
                             "issuer: %s, not valid after: %s, X.509 usage extensions: %s",
-                    stripCRLF(prefix), stripCRLF(c.getSubjectDN().getName()), stripCRLF(sans(c, ",")), stripCRLF(c.getIssuerDN().getName()),
+                    stripCRLF(prefix), stripCRLF(c.getSubjectX500Principal().getName()), stripCRLF(sans(c, ",")), stripCRLF(c.getIssuerX500Principal().getName()),
                     c.getNotAfter(), stripCRLF(extensions(c)));
         } catch (Exception e) {
             return "Error while retrieving " + prefix + " certificate information";