[CI] Upgrade vllm commit hash

[Potabk]

What this PR does / why we need it?

Upgrade vllm commit hash to a2981c42720a34b5abf59c4c14df701f8105d4cd

Does this PR introduce any user-facing change?

How was this patch tested?

• vLLM version: v0.11.0
• vLLM main: vllm-project/vllm@2918c1b

[github-actions]

👋 Hi! Thank you for contributing to the vLLM Ascend project. The following points will speed up your PR merge:‌‌

• A PR should do only one thing, smaller PRs enable faster reviews.
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• Write the commit message by fulfilling the PR description to help reviewer and future developers understand.

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[gemini-code-assist]

Code Review

This pull request updates the vLLM commit hash for the main branch in the versioning policy documentation. While the change itself is straightforward, I've identified a potential maintainability issue. The commit hash, which seems to be related to CI configuration based on the PR title, is hardcoded. This could lead to inconsistencies between the documentation and the actual CI configuration. I've left a comment with a suggestion to use a substitution variable from conf.py to act as a single source of truth, which would improve the robustness of this configuration.

[Potabk]

@wangxiyuan @MengqingCao I'd like to use feature vllm-project/vllm@a2981c4, thank you very much

[MengqingCao]

LGTM as all CI passed

[github-actions]

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