Fix KCMs common license from LGPL-2+3 to LGPL-2.1+3
Dear @ahmadsamir, you licensed the 2 files kcms/kcms-common.cpp
and kcms/kcms-common_p.h
under LGPL-2.0-only OR LGPL-3.0-only OR LicenseRef-KDE-Accepted-LGPL.
This looks strange because it skips LGPL-2.1 between 2.0 and 3.0, and is inconsistent with all other stated licenses in plasma-workspace
which use LGPL-2.1-only OR LGPL-3.0-only OR LicenseRef-KDE-Accepted-LGPL.
So I guess this was not intentional and this MR fixes it.
Cheers,
Aurélien