MNT: Use dependabot to auto update workflow dependencies

[pllim]

I noticed that some of your workflows use outdated stuff. This would keep it up-to-date going forward. After merge, the bot will open PR to update.

https://docs.github.com/en/code-security/getting-started/dependabot-quickstart-guide

[lwasser]

@all-contributors please add @pllim for code, review

[allcontributors[bot]]

@lwasser

@pllim already contributed before to code, review

[lwasser]

@pllim this is really really helpful. i know our workflows do become out of date. if this works as i think it might, then we can add it to our other repos too! thank you!!