shell/desktopview: Change preview banner title
By making it shorter, it more comfortably fits with the text:
| Version | Before | After |
|---|---|---|
| 5.27.80 | Plasma 5.27.80 Development Build | KDE Plasma 5.27.80 |
| 5.27.90 | Plasma 5.27.90 Beta Build | KDE Plasma 6.0 Beta |
| 5.27.91 | Plasma 5.27.91 Beta Build | KDE Plasma 6.0 Beta 2 |
Edited by Oliver Beard