bump `flashinfer` to `v0.2.8` (#21385)

Signed-off-by: cjackal <44624812+cjackal@users.noreply.github.com>

docker/Dockerfile

@@ -390,7 +390,7 @@ RUN --mount=type=bind,from=build,src=/workspace/dist,target=/vllm-workspace/dist
 
 # Install FlashInfer from source
 ARG FLASHINFER_GIT_REPO="https://github.com/flashinfer-ai/flashinfer.git"
-ARG FLASHINFER_GIT_REF="v0.2.8rc1"
+ARG FLASHINFER_GIT_REF="v0.2.8"
 RUN --mount=type=cache,target=/root/.cache/uv bash - <<'BASH'
   . /etc/environment
     git clone --depth 1 --recursive --shallow-submodules \