fdo#72157: webdav: Release WebDAV file lock on document close.

Make it work with both serf and neon version.

Change-Id: I3c74ba57cde19f4417d0229c1c809756e3c5f9f8
(cherry picked from commit 956d088db5850e9a096bb9abd8b3e446e096f613)
Reviewed-on: https://gerrit.libreoffice.org/9880
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>