Change `checkout` actions to use newer `restore` syntax

[jat255]

This PR resolves #691 and changes all references to git checkout to either git restore (or git switch) as appropriate. I believe I got all the references, but a close look at the changes my a maintainer would be great. Since 2.28.0 is the default version in newer versions of Git for Windows and Linux, I think it makes sense to make these changes now. I've included the older syntax in comments in case people are using an older version, too.

I also tried to update the output blocks to reflect the verbiage used by git in more modern versions (I ran all the examples with Git 2.28.0 on Linux).