Update major versions of Actions in README and workflows

[henrymercer]

Update the README to use the latest major versions of actions/checkout and the Actions in this repo. Also update our workflows to use the latest major versions of various first-party Actions.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[cklin]

Thank you for the change!

@henrymercer
A question just popped up in my mind. Do we want these documentation changes in the v1 branch as well? I can see arguments both ways...

[henrymercer]

Thank you for the change!

@henrymercer A question just popped up in my mind. Do we want these documentation changes in the v1 branch as well? I can see arguments both ways...

Good question. For now I don’t think I see a good enough reason for the branches to diverge. But very happy to reconsider.