GH Actions: version update for various predefined actions

[jrfnl]

Summary

This PR can be summarized in the following changelog entry:

• Improve CI

Relevant technical choices:

A number of predefined actions have had major release, which warrant an update 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

Test instructions

This PR can be tested by following these steps:

• N/A If the build passes, we're good.