Auto-merge dependabot updates passing tests

[llucax]

Proposal

1. Set up a workflow to run on every PR and sets the auto-merge option.
2. Set up a workflow that runs on a schedule (daily?) that looks for open PRs created by dependabot and approves it iff it is older than X days (7?) and unless, the status:blocked label is set.

The effect of this is that we don't need to care about dependabot PRs anymore, eventually they will be auto-merged, unless there is something fishy about it, in which case we can just go and have a look, and flag it to not be merged automatically with a label.

References

• https://docs.github.com/en/code-security/dependabot/working-with-dependabot/automating-dependabot-with-github-actions
• There is also the ahmadnassri/action-dependabot-auto-merge action, but not sure if it makes sense now that GitHub provides "official" tools to do it.