Switches the dependency-updates workflow to use HOMEBREW_GITHUB_API_TOKEN
(the same PAT used to bump the homebrew formula on release) and sets
the committer/author to Dokku Bot <no-reply@dokku.com>.
Bumps every entry in contrib/dependencies.json to its latest upstream
release. Adds a scheduled GitHub Actions workflow that checks each
dependency once a day and opens one PR per outdated entry.