Apple dropped support for some older platforms, so we can also remove
the annotations for them.
See https://developer.apple.com/support/xcode/ for the supported
versions.
This patch introduces the support::make_test_jthread utility which is
basically the same as support::make_test_thread but for std::jthread. It
allows vendors to maintain a downstream way to create threads for use
within the test suite, which is especially useful for embedded
platforms.
