No `checkout`

[martinosorb]

This removes the last instances of git checkout: two cases that had been missed, in which checkout was still included in the suggestions of git status (this is now revert). The other was a change from git checkout -b to git switch -c, not covered by the previous pull request, which dealt with changing the cases that could be turned into git revert.

[github-actions[bot]]

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[martinosorb]

Note: we should add a comment in the instructor notes that explains this and offer alternatives in case of older versions of git (Toby's suggestion).