docs: fix inconsistent setup instructions in travel-concierge README: use uv instead of poetry

[mauripsale]

This PR fixes inconsistent setup and execution instructions in the travel-concierge sample.

Changes:

• Removed obsolete references to poetry env activate in the installation steps.
• Aligned documentation with the project's use of uv (as evidenced by the existing uv.lock file).
• Updated the "Running the Agent" section to recommend the uv run workflow.
• Standardized command examples to use uv run adk ... for better developer experience and reliability.

Motivation

Currently, the README directs users to install dependencies with uv sync but then attempts to activate a non-existent Poetry environment. This creates a "breaking point" for new developers trying to run the sample. These changes ensure a smooth, end-to-end setup process using the modern `u toolchain already adopted by the repository.

CLA

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