Add support for Dependabot PRs - automated-security-fixes

[jboursier-mwb]

The automated-security-fixes key is not supported by this provider which prevents Dependabot PRs to get enabled.

vulnerability_alerts only enable alerts, while the former key enables associated PRs to be enabled.

• REST API endpoint: https://api.github.com/repos/%7Borganization%7D/%7Brepository%7D/automated-security-fixes
• REST API documentation: https://docs.github.com/en/rest/repos/repos?apiVersion=2022-11-28#enable-automated-security-fixes

[jboursier-mwb]

Duplicates integrations/terraform-provider-github#588

Closing, sorry for the noise.