kconfig_update for auto border size introduction

Summary:
If a user has set his border size explicitly in the past also respect this
selection with the new auto border size mode by setting auto borders to
false on update.

For this a kconf_update file and a sed based shell script is added.

Test Plan: Manually.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: zzag, davidedmundson, kwin

Tags: KWin

Maniphest Tasks: T8707

Differential Revision: https://phabricator.kde.org/D13480