GHA: switch to Checkout V3 to prevent node.js nagging

[indigoxela]

Recently GitHub Actions switched to require node.js 16:

https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/

We're still on actions/checkout@v2

And that's why we get:

Easy to solve: simply switch to actions/checkout@v3.

[indigoxela]

A PR is available for review.

[klonos]

Thanks @indigoxela (for this issue specifically, and for taking care of many of our GHA-related tasks) 🙏🏼

I've had a look at the code, and it looks good to me + tests are passing, so RTBC 👍🏼

[quicksketch]

Merged https://github.com/backdrop/backdrop/pull/4268 into 1.x and 1.23.x. Thanks @indigoxela and @klonos!