Update deprecating ubuntu 18.04

[adrian-kong]

Description

For deprecating ubuntu 18.04 not needed for like another 4 months
actions/runner-images#6002

[silverjam]

Description

For deprecating ubuntu 18.04 not needed for like another 4 months actions/runner-images#6002

Ooops, hit the wrong button, didn't mean to close the PR... for gcc-6, we're going to have to switch to docker for that step, or just drop support for gcc-6 here (it'll be implicitly checked elsewhere).

For example: https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#example-using-a-docker-hub-action

[adrian-kong]

Superseded by #1250