kconf_update: Also migrate from BreezyGTK

Summary: Also recognize BreezyGTK as theme name to migrate from.

Test Plan:
Was applied downstream for years.
Apparently nobody sent that upstream, so let's fix that.

Reviewers: Plasma, ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22237