Upgrade actions/checkout to v3

[cbush]

Checking if v3 supports the pull_request_target event to actually check out the PR branch by default

[mongodben]

based on the docs for v3, seems like it'd get the HEAD commit for the branch that triggers the workflow. relevant documentation:

i also see this subsection of docs saying that that you can explicitly specify the 'pull request HEAD commit instead of merge commit' - https://github.com/actions/checkout#checkout-pull-request-head-commit-instead-of-merge-commit
not sure what this means but seems as if it might be relevant.

also, are there any expected changes to behavior of the GH actions?

[cbush]

Exactly why I didn't add the second snippet ("PR head").

Everything was working as expected until we started using the (relatively new) pull_request_target event.

Didn't notice @nathan-contino-mongo's issue because our tests didn't actually modify the docusaurus source in any meaningful way.

So, my hypothesis is that <v3 just doesn't handle pull_request_target correctly. By upgrading, I'm hoping it will go back to the way it says it works in the docs, which is that by default (i.e. without the ref option) the checked out branch is the PR ref.

I don't expect any impact on the other workflows.

[mongodben]

lgtm.

we should prob test out the pull_request_target action further as well.

[nathan-contino-mongo]

LGTM

[cbush]

Boo, turns out my hypothesis was incorrect! Going with solution here: actions/checkout#518 (comment)

Since we want the same "default" behavior of checkout on the pull_request event, we're checking out the merge commit.