Migrate away from "@dependabot squash and merge"

[caugner]

See: https://github.blog/changelog/2025-10-06-upcoming-changes-to-github-dependabot-pull-request-comment-commands/

Beginning on January 27th (…), these comment commands will no longer be supported.

[caugner]

In mdn/rumba, I noticed this error following the migration:

GraphQL: Resource not accessible by personal access token (enablePullRequestAutoMerge)

The explanation was that the PAT only had pull-requests: write permission, but not content: write.