chore: update guzzle-cache-middleware from 4.x to 7.0

[kinyoklion]

Summary

Update kevinrob/guzzle-cache-middleware from 4.* to ^7.0 to fix PHP 8.4+ deprecation warnings about implicit nullable parameters.

The LD PHP SDK uses guzzle-cache-middleware via GuzzleFeatureRequester for HTTP response caching in polling mode. The v7 API is backward-compatible — CacheMiddleware, PublicCacheStrategy, and Psr6CacheStorage all retain the same interfaces.

Review & Testing Checklist for Human

• Run composer install && php main.php and verify no deprecation warnings appear
• Verify flag evaluation still works correctly

Notes

Tested with PHP 8.4 via Docker — no deprecation warnings with v7, app evaluates flags correctly.

Link to Devin session: https://app.devin.ai/sessions/b648d8cd5e0b4a04ba0cbf1dcd1a9886
Requested by: @kinyoklion

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