drop internal os-release class in favor of KOSRelease of kcoreaddons 5.58

Summary:
KOSRelease was based on our class so it is fully API compatible and doesn't
require any changes besides different include and name

Test Plan: builds + modules still loads os release data

Reviewers: davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21209