[RunAllTests] Fix #4512: Upgrade CI to Ubuntu 20.04

[BenHenning]

Explanation

Fixes #4512

Per actions/runner-images#6002 this migrates all CI actions over to Ubuntu 20.04 since 18.04 has been deprecated and is being phased out of production.

Due to limited time, I'm planning to fast-track this PR by merging it if CI passes for all tests.

Essential Checklist

• The PR title and explanation each start with "Fix #bugnum: " (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• Any changes to scripts/assets files have their rationale included in the PR explanation.
• The PR follows the style guide.
• The PR does not contain any unnecessary code changes from Android Studio (reference).
• The PR is made from a branch that's not called "develop" and is up-to-date with "develop".
• The PR is assigned to the appropriate reviewers (reference).

For UI-specific PRs only

N/A -- This only affects the CI runtime environment, not the app or anything user-facing.

[BenHenning]

@seanlip PTAL or delegate. Note that I'll update the required workflows once this is ready to get merged since I'll need to send out a mail to the developer list given this will break all in-flight PRs and lead to a required re-sync.

[seanlip]

LGTM

[seanlip]

(Feel free to update the required workflows.)

[oppiabot]

Hi @BenHenning, this PR is ready to be merged. Please address any remaining comments prior to merging, and feel free to merge this PR once the CI checks pass and you're happy with it. Thanks!