maint: disable dependabot (#972)

## Which problem is this PR solving?

- the deluge of dependabot PRs that neglected anyway

## Short description of the changes

- remove dependabot.yml

![so long
partner](https://github.com/honeycombio/example-greeting-service/assets/29520003/c842d2ba-6539-4319-90da-91d763b9d916)

I'll be honest I've never intentionally disabled dependabot altogether
so i'm not sure if there's more to it than removing this file. But these
dependabot PRs clog up the PRs in this repo and in our notifications. We
generally update a whole service at once anyway so it makes more sense
to keep doing that.