GH Actions: version update for various predefined actions

[jrfnl]

A number of predefined actions have had major release, which warrant an update to the workflow(s).

These updates don't actually contain any changed functionality, they are mostly just a change of the Node version used by the action itself (from Node 14 to Node 16).

Refs:

• https://github.com/actions/checkout/releases
• https://github.com/actions/download-artifact/releases
• https://github.com/actions/upload-artifact/releases

[jrfnl]

Note: the build failure is unrelated to this PR, but has to do with WP Core.