Drop vLLM 0.15 support

[qgallouedec]

As per the plan

---

Note

Low Risk
Documentation and dependency bounds only; no training or vLLM integration logic changes.

Overview
Drops vLLM 0.15.x by raising TRL’s supported floor from 0.15.0 to 0.16.0 (upper bound 0.23.0 unchanged).

The vLLM integration warning in docs/source/vllm_integration.md, the vllm extra in pyproject.toml (vllm>=0.16.0,<=0.23.0), and is_vllm_available() in trl/import_utils.py (range check and user warning text) are all aligned to the new minimum.

^{Reviewed by Cursor Bugbot for commit d702318. Bugbot is set up for automated code reviews on this repo. Configure here.}

[bot-ci-comment]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.