Upgrade `actions/checkout` in CI

[peterstace]

Description

In particular, this uses Node 16 rather than Node 12 (which has been deprecated on Github).

Check List

Have you:

• Added unit tests? N/A

• Add cmprefimpl tests? (if appropriate?) N/A

• Updated release notes? (if appropriate?) N/A

Related Issue

• https://github.com/peterstace/simplefeatures/issues/499

[peterstace]

Thanks for reviewing!