message: Add Debian 11 "bullseye" to the officially supported list
diff: iTwin.js **backends** are built and tested on the following:
  -- Debian 9 "Stretch" and Debian 10 "Buster"
  +- Debian 10 "Buster" and Debian 11 "Bullseye"
  - Windows 10 version 1803 (or greater)
  - MacOS 10.15 (or greater)
  