Update Github Actions dependencies

[fhanau]

• This finally fixes warnings about the deprecated node16 being used for CLAssistant.
• Use short-form for dependencies in BUILD.dawn

[anonrig]

Should we also enable dependabot?

[fhanau]

Should we also enable dependabot?

IMHO dependabot can just be spamming PRs sometimes... We don't use GitHub Actions dependencies that much and managing dependencies is quite sane for them in the general case (e.g. we use actions/checkout@v4 which maps to the latest available 4.x release, so we get non-major updates automatically). These dependencies have been mostly low-maintenance and the current approach of updating them as needed/based on deprecation warnings works well enough.

[anonrig]

IMHO dependabot can just be spamming PRs sometimes...

Agreed. In terms of security, if we are not updating them regularly, it might be better to use commit hashes instead of semver versions for github action dependencies.