[FEATURE] Update flagd-testbed (test-harness)

[mmito]

Requirements

Currently, the integration tests are running on an 1.5 year-old version of the flagd-testbed.

Since then the OpenFeature Test Harness repo has been moved to a different flagd-testbed repository, but integration tests have not been updated here.

The latest flagd-testbed offers support for recent versions of flagd (currently v.0.11.3), whilst here we are stuck testing with flagd v.0.4.1. The newer versions patched a variety of issues, some of which are not caught by the current openfeature/php-sdk integration tests (e.g.: missing "value" key in the evaluation response when using string targetingKey instead of key-value Attributes -> here).