License: Restore long lost OpenSSL exception (TrinityCore#29250)

Restore long lost OpenSSL exception accidentally removed in 4559e99 and include it in the COPYING license file.

COPYING

@@ -337,3 +337,14 @@ proprietary programs.  If your program is a subroutine library, you may
 consider it more useful to permit linking proprietary applications with the
 library.  If this is what you want to do, use the GNU Lesser General
 Public License instead of this License.
+
+                            OpenSSL exception
+In addition, as a special exception, the TrinityCore project
+gives permission to link the code of its release of Trinity Core with
+the OpenSSL project's "OpenSSL" library (or with modified versions of
+it that use the same license as the "OpenSSL" library), and distribute
+the linked executables.  You must obey the GNU General Public License
+in all respects for all of the code used other than "OpenSSL".  If you
+modify this file, you may extend this exception to your version of the
+file, but you are not obligated to do so.  If you do not wish to do
+so, delete this exception statement from your version.