Change validator for Endpoint Address entry field

Summary:
BUG: 408670

Change validator for FQDN entry in Endpoint Address entry
field to allow single character domain names rather than
limiting them to a minimum of two characters.

Reviewers: jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22283