release: 0.0.8 (#282)

Bumps `langchain-weaviate` from 0.0.7 → 0.0.8.

## What's included since v0.0.7
- Async support via optional `WeaviateAsyncClient` (#233)
- Telemetry header change: `langchain` → `langchain-python` (#279)
- Dependency bumps (#278, #280)

## Release
Version-only bump in `libs/weaviate/pyproject.toml`. After merge,
trigger the `release` workflow (`workflow_dispatch`, `working-directory:
libs/weaviate`) to publish to PyPI and cut the `libs/weaviate/v0.0.8`
tag.