Use Ubuntu 20.04 artifacts for various release processes

From !3894 (closed)

1. Use Ubuntu 20.04 as base image for our Docker image
2. Update other release processes to use Ubuntu 18.04 artifacts.

From a quick glance, the changes needed are

1. 20.04 builds should happen before docker builds. Revert !3741 (merged).
2. Triggered package builds should build 20.04 packages - https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/gitlab-ci-config/gitlab-com.yml#L251
3. Getting GitLab Rails's SHA should use focal - https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/lib/gitlab/build/qa.rb#L30
4. The artifacts used for Docker and AMI builds must be from focal - https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/lib/gitlab/build/info.rb#L136 and https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/lib/gitlab/build/info.rb#L164
5. AWS AMIs should be built from 20.04 images to use these new artifacts - https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/master/support/packer/ee.json#L13, https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/master/support/packer/ee-ultimate.json#L14, and https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/master/support/packer/ce.json#L13 (Note: This may change by !3841 (merged))
6. Other instances where we hardcode xenial or 16.04