lib: cpp: Rename CONFIG_CPLUSPLUS to CONFIG_CPP

There is no reason for the top-level C++ Kconfig symbol to be called
`CPLUSPLUS` when the convention used throughout the Zephyr codebase is
to use "cpp" to refer to C++ and the rest of the C++-related Kconfig
symbols are prefixed with `CPP_`.

This commit renames the Kconfig `CPLUSPLUS` symbol to `CPP` in order to
better align with the de-facto standard naming convention used throughout
the Zephyr codebase, and deprecates the old `CPLUSPLUS` symbol.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>