Use KSysGuard namespace for KSysGuard targets

Summary:
With D29302 all KSysGuard targets have been placed in the KSysGuard namespace
and the KF5 versions deprecated. This changes KSysGuard to use the new
namespace. In addition, it renames the libkstats target from "PW5::KSysGuardBackend"
to "KSysGuard::StatsBackend".

Test Plan: Things still compile.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29507