testsuite: Disable tests for C++23 that depend on std::unexpected

These tests depend on unexpected handlers, which are no longer declared
for C++23 mode. Adjust the target specifier so they don't run.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/noexcept06.C: Disable for C++23.

libstdc++-v3/ChangeLog:

	* testsuite/18_support/exception/38732.cc: Disable for C++23.
	* testsuite/18_support/headers/exception/synopsis.cc: Likewise.
	* testsuite/18_support/unexpected_handler.cc: Likewise.