Raspberry: replace raspbian with ubuntu?

[fnoop]

Triggered by #1005, should we replace raspbian with ubuntu? Raspbian is the official raspberry distro and so is very well supported and understood, and provides the best hardware support.

• Raspbian name replaced with Rasperry Pi OS
• Up to now only 32bit (armv7l), but 64bit OS (aarch64) in beta
• Ubuntu will run normally in 64bit (aarch64)
• Raspbian now reports as raspbian rather than debian which upsets quite a lot of build/autodetect systems
• Ubuntu generally speaking much better supported
• Raspberry hardware better supported by Raspbian