[flagd] Update selector handling for consistency with sync, evaluation, and OFREP services

[aepfli]

Context

Refer to open-feature/flagd#1814 and the related implementation change to selector handling across flagd services.

Requirements

• Document migration path for users of java-sdk-contrib with respect to selector normalization, specifically calling out changes to usage of flagd-selector header
• Update SDK and plugin usage samples to reflect the preferred approach
• Add deprecation notice to any examples or docs that still use request body selector

Tasks

1. Update user-facing documentation and guides for selector normalization
2. Ensure SDK/plugin samples use the new preferred header
3. Add a section on maintaining backward compatibility and preparing for future major version breaking change

Additional notes

• Work should align with documentation approaches used in core flagd
• Link to upstream issue: [FEATURE] Normalize selector handling across sync, evaluation, and OFREP services flagd#1814