[effects/cube] Update cubecap texture

Summary: Current cubecap texture is an old KDE logo. Let's use something newer.

Test Plan:
Before:
{F6282237}

After:
{F6282239}

Before:
{F6282372}

After:
{F6282375}

Reviewers: #kwin, #plasma, #vdg, davidedmundson

Reviewed By: #kwin, #plasma, davidedmundson

Subscribers: davidedmundson, ngraham, broulik, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15726