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 automation config only; no application runtime or auth/data paths change.

Overview
Adds a new Dependabot configuration so GitHub Actions dependencies under / are checked weekly, with a cooldown.default-days: 7 on routine version updates (security updates are unaffected per Dependabot behavior).

This aligns the repo with org standards (SEC-8058) to space out non-security dependency PR noise while keeping weekly discovery.

^{Reviewed by Cursor Bugbot for commit a2d6c95. Bugbot is set up for automated code reviews on this repo. Configure here.}

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