Update `actions/checkout` to @v3

[zimeg]

Summary

This PR bumps the version of actions/checkout to @v3 to prevent warnings of using an outdated Node version in @v2.

Example warning:

Related links:

• https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/
• https://github.blog/changelog/2023-07-17-github-actions-removal-of-node12-from-the-actions-runner/

[zimeg]

Realizing that warning is from a workflow in my own repo... I had initially thought it was being raised from this action! My bad!

No hard feelings if this PR should be closed!