ci: fix vllm-cu13 base image default to unsuffixed :latest tag (#2140)

## Summary
The `nixl-ci-build-llm-container` pipeline's vLLM CU13 build defaulted
to `docker.io/vllm/vllm-openai:latest-cu130`, which is no longer valid.
Starting with vLLM v0.20.0, CUDA 13 became the default and current CUDA
13 images are published under the unsuffixed `vllm/vllm-openai:latest`
tag.

- `.ci/jenkins/lib/build-llm-container-matrix.yaml`: default
`BASE_IMAGE_VLLM_CU13` fallback changed to
`docker.io/vllm/vllm-openai:latest`; updated surrounding comments
- `.ci/jenkins/pipeline/proj-jjb.yaml`: updated the
`BASE_IMAGE_VLLM_CU13` job parameter description to match

CU12 (`latest-cu129`) remains unchanged, as it's still a valid CUDA 12.9
tag.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Chores**
* Updated the CUDA 13 vLLM base image reference to use the current
`latest` tag.
* Updated release-candidate build configuration to reference the
corresponding vLLM image.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: NirWolfer <nwolfer@nvidia.com>