Re-enable dependabot on `main` branch

[dstansby]

It looks like dependabot is only running on the default branch, which is currently v3. This means that we're missing compatability issues with dependencies because dependabot is no longer updating the test dependencies on the main branch, e.g., see #1891 and #1679.

I think a repository admin will need to configure dependabot in the repository settings to run on both branches.

[dstansby]

Looking at the dependabot config options, there isn't a way to get it running on a non-default branch 😢

[dstansby]

I spoke too soon, there is target-branch!