Drop vLLM 0.15 support

[qgallouedec]

As per the plan

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Note

Low Risk
Documentation and dependency bounds only; no runtime integration logic changed beyond aligning the existing version warning.

Overview
Raises the minimum supported vLLM version from 0.15.0 to 0.16.0 (upper bound stays 0.23.0).

The supported range is updated in three places: the vLLM integration doc warning, the trl[vllm] optional dependency in pyproject.toml, and the version check plus warning text in is_vllm_available() in trl/import_utils.py. Users on vLLM 0.15.x will see the compatibility warning and pip install "trl[vllm]" will no longer resolve to 0.15.

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