pgsql: 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

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/aae9aad19d824af72640f4597a812d75ae25e385

Modified Files
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.cirrus.tasks.yml             | 3 ---
src/include/utils/pg_locale.h | 5 +++++
2 files changed, 5 insertions(+), 3 deletions(-)