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
P.S.: looks like there are some more actions in use which have had new major releases. As those actually _do_ contain changed functionality, I've excluded the others from this PR.
Alex