Keep GitHub Actions up to date with GitHub's Dependabot

[cclauss]

https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot

Like #2211 but automated.

• chore: upgrade actions to v4 #2211

Description

Please describe what changes you made, and why you feel they are necessary. Make sure to include
code examples, where applicable.

Corresponding issue:

Testing

Please explain how you tested this change manually, and, if applicable, what new tests you added. If
you're making a change to just the website, you can omit this section.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded

[alexpetros]

I'm okay with the manual process; GitHub actions aren't so important that we need an additional GitHub action to update our GitHub actions.