chore: Add Dependabot version-update cooldown

[ld-repository-standards]

This pull request was auto generated by the LaunchDarkly Github Standards automation platform.

• Ensure every entry under updates in .github/dependabot.yml declares a cooldown of at least 7 days (default-days).
• Add entries for detected package ecosystems that were not yet tracked by Dependabot.

Cooldown applies only to version updates; security updates bypass it, so critical CVE fixes are never delayed.

Ref: SEC-8058.

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Note

Low Risk
CI/dependency automation only; no application runtime or security logic changes.

Overview
Introduces .github/dependabot.yml so Dependabot tracks github-actions and gomod at the repo root on a weekly schedule.

Each ecosystem now sets cooldown.default-days: 7, spacing routine version-update PRs; security updates still bypass cooldown per Dependabot behavior.

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