chore(deps): bump vllm from 0.26.0 to 0.28.0 in /backend/python/vllm (#11752)

Bumps [vllm](https://github.com/vllm-project/vllm) from 0.26.0 to 0.28.0.
- [Release notes](https://github.com/vllm-project/vllm/releases)
- [Changelog](https://github.com/vllm-project/vllm/blob/main/RELEASE.md)
- [Commits](https://github.com/vllm-project/vllm/compare/v0.26.0...v0.28.0)

---
updated-dependencies:
- dependency-name: vllm
  dependency-version: 0.28.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

backend/python/vllm/requirements-l4t13-after.txt

@@ -9,4 +9,4 @@
 # memory architecture crash deterministically with an empty "Engine core init
 # failed" set (mudler/LocalAI#10722). Leaving this unpinned let the L4T image
 # drift onto whatever wheel was latest at build time.
-vllm==0.26.0
+vllm==0.28.0