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

[dpward]

Ubuntu 16.04 LTS reached the end of standard support 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.

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 is a paid image that includes ESM.