fix(github): dependabot config (#10088)

### Changes 🏗️

The changes on my recent PR,
https://github.com/Significant-Gravitas/AutoGPT/pull/10072 , moving from
`yarn1` to `pnpm` in the FE [made dependabot break
](https://github.com/Significant-Gravitas/AutoGPT/runs/43465590584)😭

I forgot, dependabot implicitly supports pnpm:
https://github.com/Significant-Gravitas/AutoGPT/pull/10087/files ( _you
can see it updated the `pnpm-lock.yml` file correctly_ ) but you need to
set the `dependabot.yml` config with `npm` as the package ecosystem.

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan: