Add allow list to git dependabot config

We might want to add "allow" to the git dependabot configuration.

I'd leave it open for a few hours (or days) and then merge even if @mcfnord doesn't add it. Otherwise, we should open an issue.

_Ursprünglich gepostet von @ann0see in https://github.com/jamulussoftware/jamulus/pull/3848#issuecomment-5284270412_

Locate the repository's Dependabot configuration and read the existing dependency update settings first. Add the requested allow-list configuration, then verify that the configuration is valid and limits updates as intended; the issue does not name a file or test to run.

Written by the indexing model from the issue text.