Bump Raspberry Pi Ubuntu Server version to 21.04 (#7650)

Ubuntu 20.10 is out of support and furthermore, the Raspberry Pi imaging
tool no longer includes this version. Bump the version to the current
one (21.04). Either version should work fine.

docs/BUILDING.md

@@ -73,9 +73,9 @@ dependency.
 
 #### How to install prerequisites on Raspberry Pi 4
 
-Using `rpi-imager`, install the Ubuntu _20.10_ LTS 64-bit _server_ OS for arm64
-architectures on a micro SD card. This release will have bluez 5.55 which is
-required for BLE functionality.
+Using `rpi-imager`, install the Ubuntu _21.04_ 64-bit _server_ OS for arm64
+architectures on a micro SD card. This release will have bluez 5.55 or newer
+which is required for BLE functionality.
 
 Boot the SD card, login with the default user account "ubuntu" and password
 "ubuntu", then proceed with "How to install prerequisites on Linux".