Update README.md

[damieng]

Move VectorSearch to supported features and extend it out in the README.

[Copilot]

Pull Request Overview

This PR updates the README documentation to reflect that Vector Search is now a supported feature. The change moves Vector Search from the "not yet supported" list to the "supported features" list, with additional details about the VectorSearch extension method and fluent vector index configuration.

Key Changes

• Added Vector Search to the supported features section with implementation details
• Removed Vector Search from the unsupported features list

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