Rename Tramp method "owncloud" to "nextcloud"

Rename Tramp method "owncloud" to "nextcloud"

In the spirit of freedom, "nextcloud" is preferred over "owncloud".

* doc/misc/tramp.texi (Quick Start Guide, GVFS based methods):
* etc/NEWS: Rename "owncloud" method to "nextcloud".

* lisp/net/tramp-gvfs.el (tramp-gvfs-methods, tramp-goa-methods)
(tramp-gvfs-url-file-name, tramp-gvfs-handler-mounted-unmounted)
(tramp-gvfs-connection-mounted-p, tramp-gvfs-mount-spec)
(tramp-get-goa-accounts): Use "nextcloud" instead of "owncloud".
(tramp-gvfs-nextcloud-default-prefix)
(tramp-gvfs-nextcloud-default-prefix-regexp): Rename them.  Adapt
all callees.

* test/lisp/net/tramp-tests.el (tramp--test-nextcloud-p):
Rename from `tramp--test-owncloud-p'.
(tramp-test11-copy-file, tramp-test12-rename-file): Use it.