Adopt use of Mend Renovate App to automatically update tooling and dependencies

[ni-jfitzger]

Description of issue

• We periodically have to update our use of GItHub actions in workflows due to deprecations
• Sometimes a dependency for a tox command will break due to a new version of an external dependency and we won't discover it until our next PR
• We currently don't lock down our dependencies used to build documentation (so our documentation build could break without warning), but if we do lock them down then we'll accumulate tech debt

The Mend Renovate app can help with this by automatically creating PRs to update actions and dependencies.

Mentioned by @bkeryan in #2039

[ni-jfitzger]

We should read up on the app and its terms of service to make sure it's right for us.

[ni-jfitzger]

Worth considering as an alternative: GitHub's dependabot.
They essentially do the same thing.