CI upgrade to ubuntu-20.04

[ringabout]

The ubuntu-18.04 environment is deprecated, consider switching to ubuntu-20.04(ubuntu-latest), or ubuntu-22.04 instead. For more details see https://github.com/actions/virtual-environments/issues/6002

[ringabout]

It seem to be the time we have to drop 32 bits linux CI support.

Azure pipelines is about to remove ubuntu 18.04 os image, but ubuntu 20.04 doesn't support i386 processor. We have no choice but to remove the i386 matrix.

[ringabout]

According to https://github.com/actions/runner-images/issues/6002#issuecomment-1211855838, we can probably use container for i386 ubuntu 18.04.

https://hub.docker.com/r/i386/ubuntu/

I will fix it when it is necessary.

[github-actions[bot]]

Thanks for your hard work on this PR! The lines below are statistics of the Nim compiler built from 8155837cde99d7d532071a2a56d95485ffb79292

Hint: mm: orc; threads: on; opt: speed; options: -d:release 163645 lines; 11.818s; 841.684MiB peakmem