Rename COPYING-AGPL to COPYING

That way GitHub properly indexes the license information and displays it inline. Also renamed one old usage of "ownCloud" to Nextcloud.

COPYING-README

-Files in ownCloud are licensed under the Affero General Public License version 3,
-the text of which can be found in COPYING-AGPL, or any later version of the AGPL,
+Files in Nextcloud are licensed under the Affero General Public License version 3,
+the text of which can be found in COPYING, or any later version of the AGPL,
 unless otherwise noted.
 
 Licensing of components:
@@ -12,4 +12,4 @@ Licensing of components:
 All unmodified files from these and other sources retain their original copyright
 and license notices: see the relevant individual files.
 
-Attribution information for ownCloud is contained in the AUTHORS file.
+Attribution information for Nextcloud is contained in the AUTHORS file.