meson.build: Refuse XCode versions < v15.0

meson.build: Refuse XCode versions < v15.0

According to our support policy, we only support the two latest
major versions of macOS, and we already removed compatibility code
for older versions. However, it's still possible that people install
an older version of XCode on a recent version of macOS - which won't
be able to compile QEMU anymore, see for example the ticket here:

 https://gitlab.com/qemu-project/qemu/-/issues/2694

Thus let's set the expectations right and refuse older versions of
XCode that do not match the two latest versions of macOS anymore.

Message-ID: <20241126081054.244365-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>