[sftp] use UDSEntry::fastInsert instead of deprecated insert

Summary:
this bumps the requirement of kf5 to 5.48, by the time this gets released
(18.12) 5.48 should be sufficiently adopted out in the wild

Test Plan:
- looked at the code
- ran copy/read/write/list/refresh without problemos

Reviewers: broulik

Reviewed By: broulik

Differential Revision: https://phabricator.kde.org/D15499