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.

---

Note

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

Overview
Updates .github/dependabot.yml so every updates block includes cooldown.default-days: 7, spacing out routine version bumps while security updates still bypass cooldown.

The existing /getting-started/HelloDotNet NuGet entry gains that cooldown on its daily schedule. New entries cover GitHub Actions at repo root and NuGet under several server-sdk-ai feature/framework paths plus server-sdk/getting-started/HelloDotNet, mostly on a weekly schedule with the same 7-day cooldown.

^{Reviewed by Cursor Bugbot for commit 3b666a3. Bugbot is set up for automated code reviews on this repo. Configure here.}