Upgrade github actions to Node 20

[ferhoyos]

Currently, github actions use v3 of actions/checkout, which internally runs on Node 16 (https://github.com/actions/checkout/blob/v3/action.yml#L90).

This PR upgrades github actions to v4, which internally runs Node on 20 (https://github.com/actions/checkout/blob/v4/action.yml#L102)