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-only Dependabot configuration; no application runtime or security logic changes.

Overview
Introduces .github/dependabot.yml so dependency and action updates are automated on a weekly schedule.

Every updates entry sets cooldown.default-days: 7, spacing out routine version bumps. Coverage includes github-actions at the repo root, Gradle under the Flutter example Android paths, and pub for the monorepo root plus apps (flutter_client_contract_test_service, sse_contract_test_service) and packages (common, common_client, event_source_client, flutter_client_sdk, and its example).

Security updates are not subject to this cooldown.

^{Reviewed by Cursor Bugbot for commit 1851f0b. Bugbot is set up for automated code reviews on this repo. Configure here.}

[pkaeding]

via LD Research 🤖

[kinyoklion]

Needs to be restricted in update type.
PR title need a conventional commit.

"ci: Add..."