[Benchmark] Upgrade benchmark args for new vllm version

[Potabk]

What this PR does / why we need it?

Since the newest vllm commit has deprecated the arg --endpoint-type, we should use --backend instead

Does this PR introduce any user-facing change?

How was this patch tested?

• vLLM version: v0.10.2
• vLLM main: vllm-project/vllm@releases/v0.11.0

[github-actions]

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[gemini-code-assist]

Code Review

This pull request updates the benchmark configuration by replacing the deprecated endpoint_type parameter with backend in benchmarks/tests/serving-tests.json. This change aligns the benchmark tests with the latest version of vllm, which has deprecated the old argument. The modification is correctly and consistently applied across the test configurations. The change is straightforward and addresses the stated purpose of the pull request effectively.

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.