Update ICU C++ API symbols

Update ICU C++ API symbols

Recent ICU versions have added U_SHOW_CPLUSPLUS_HEADER_API, and we need
to set this to zero as well to hide the ICU C++ APIs from pg_locale.h

Per discussion, we want cpluspluscheck to work cleanly in backbranches,
so backpatch both this and its predecessor commit ed26c4e25a4 to all
supported versions.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/1115793.1754414782%40sss.pgh.pa.us
Backpatch-through: 13