Specify that the Raspberry Pi 5 SoC is the tier 1 reference target

in our list of reference targets, we simply list "Raspberry Pi 5"

https://doc.qt.io/qt-6/supported-platforms.html#support-levels-for-target-hardware

This can be misleading because in fact we actually only support the Raspberry Pi 5 SoC (System-on-Chip), whereas we don't provide support for the Raspberry Pi 5 Compute Module. I have had several customers confused by this who assume we provide support for the Compute Module and then when they create a support ticket they are surprised when support returns the ticket with unsupported hardware.

We should explicitly mention we only provide support for the Pi 5 SoC, and not the Compute Module.