Replace Ubuntu 16.04 LTS in toolchain configs (end of standard support)

Ubuntu 16.04 LTS reached the [end of standard support](https://ubuntu.com/blog/ubuntu-16-04-lts-transitions-to-extended-security-maintenance-esm) in April 2021. Updates are only provided through Extended Security Maintenance (ESM) which must be purchased from Canonical[^1]. The latest publicly-available version of Ubuntu 16.04 LTS is affected by [343 CVEs](https://ubuntu.com/16-04#:~:text=Ubuntu%2016.04%20LTS%20security%20updates).

The pre-generated toolchain configurations using Ubuntu 16.04 LTS should be changed so that they target a newer release instead. A `ubuntu1804_rbe` container definition already exists (but see also #870).

[^1]: ESM is free for personal use on up to 3 systems. For use on GCP or other public clouds, [Ubuntu Pro](https://ubuntu.com/gcp/pro) is a paid image that includes ESM.

Locate the pre-generated toolchain configurations that target Ubuntu 16.04 and compare them with the existing ubuntu1804_rbe container definition. Review #870 for related context, then update the affected configurations to target a newer Ubuntu release and verify that no 16.04 references remain.

Written by the indexing model from the issue text.