chore: update @launchdarkly/node-server-sdk minimum to 9.11.3

[kinyoklion]

Summary

Updates the @launchdarkly/node-server-sdk minimum version constraint from >= 9.0.0 to >= 9.11.3.

No deprecated APIs were found in the example code. The app builds and runs successfully with the latest SDK version.

Link to Devin session: https://app.devin.ai/sessions/cc589fb7edf04b2280685cb10c9bb431
Requested by: @kinyoklion

[devin-ai-integration]

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[devin-ai-integration]

Closing — no version bump needed. ">= 9.0.0" already resolves to the latest (9.11.3) on npm install, so the example already runs against the current SDK. Per maintainer guidance we don't raise a minimum that's already satisfied.