Add .g1t/upkeep.yml, format-compatible with Dependabot's dependabot.yml #134

Add an upkeep configuration file at .g1t/upkeep.yml on the default branch.

Done means:

• .g1t/upkeep.yml exists and uses the same schema as GitHub Dependabot's dependabot.yml (version: 2, updates list with package-ecosystem, directory, schedule and the other keys Dependabot accepts), so an existing dependabot.yml can be copied across unchanged.
• It covers the ecosystems this repository uses, found from its lockfiles and manifests.
• The file is validated against the same expected outcomes as a .github/dependabot.yml for this repository (same ecosystems, directories and schedules), with the validation recorded in the pull request.
• No .github/dependabot.yml is added unless asked for separately.

Current state: .g1t holds only brand and workflows; no upkeep.yml or dependabot.yml exists yet.