KDE Plasma: update to 5.21.3

This updates the KDE Plasma parts to release 5.21.3. There is one
new port, plasma5-systemmonitor, which is the upcoming replacement
for KDE sysguard. It is not installed by default.

Release notes are at
https://kde.org/announcements/plasma/5/5.21.3/

Previous commits tidying up DOCS options left some non-
applications consumers without the necessary kdoctools
dependency, so (re)introduce that now.