Upgrade all CI jobs to Debian bullseye

Stan Hurequested to merge
sh-update-ci-debian-bullseye into master

Debian bullseye is the new stable, so let's use that.

For the i386 job, this eliminates the need for RVM and gets rid of the SSL cert errors on Debian 9 (https://github.com/rvm/rvm/issues/5136).

Edited by Stan Hu

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