Support auto merge in Dependabot

For my private repository it's not possible to add the auto merge feature. I added dependabot via the web interface and I receive PRs but the PRs are not merged automatically.

After this did not work I also tried via the dependabot config. The behaviour is the same. The check runs daily and creates PRs but does not merge them.

```yaml
version: 1
update_configs:
- package_manager: "javascript"
directory: "/"
update_schedule: "daily"
automerged_updates:
- match:
dependency_type: "development"
# Supported dependency types:
# - "development"
# - "production"
# - "all"
update_type: "all"
# Supported updates to automerge:
# - "security:patch"
# SemVer patch update that fixes a known security vulnerability
# - "semver:patch"
# SemVer patch update, e.g. > 1.x && 1.0.1 to 1.0.3
# - "semver:minor"
# SemVer minor update, e.g. > 1.x && 2.1.4 to 2.3.1
# - "in_range"
# matching the version requirement in your package manifest
# - "all"
- match:
dependency_type: "production"
update_type: "all"
```

Start by reviewing Dependabot's web-interface setup and the YAML update_configs example, then trace how automerged_updates is handled for private repositories. Compare the reported daily PR behavior with the requested rules. Done means matching private-repository pull requests are merged automatically according to the configured dependency and update types.

Written by the indexing model from the issue text.