cmake macros: Port away from deprecated ECM var in kcoreaddons_add_plugin

The current kcoreaddons_add_plugin CMake macro causes deprecation
warnings in extra-cmake-module's KDEInstallDirs since we use an
old-style install path variable (PLUGIN_INSTALL_DIR).

The fix is simply to use KDE_INSTALL_PLUGINDIR instead.

BUG:401888
FIXED-IN:5.54

Differential Revision: https://phabricator.kde.org/D17446