[contrib] dependabot action broken

[brettmc]

https://github.com/open-telemetry/opentelemetry-php-contrib/actions/runs/15033575815/job/42251036895

It looks like dependabot now has a config change to make, and is trying to commit straight to main instead of via a PR:

remote: error: GH006: Protected branch update failed for refs/heads/main.
remote: remote: - Changes must be made through a pull request.
remote: remote: - You're not authorized to push to this branch.

cc: @cedricziel