x11/plasma5-plasma: update to 5.21.5

Tuesday, 4 May 2021. Today KDE releases a bugfix update to KDE Plasma 5, versioned 5.21.5.

Plasma 5.21 was released in February 2021 with many feature refinements
and new modules to complete the desktop experience.

This release adds a month's worth of new translations and fixes from
KDE's contributors. The bugfixes are typically small but important and
include:

Fix crash when stopping PipeWire streaming.
Fix crash on drag-and-drop over panel.
Lockscreen: also activate lock screen windows.
Platforms/drm: Fix crash in EglGbmBackend::presentOnOutput().

Full announcement:
https://kde.org/announcements/plasma/5/5.21.5/