LangChain 1.5.0 drops Python 3.10 and adds model router middleware

LangChain published version 1.5.0 of its `langchain` package, raising the minimum supported Python version to 3.11 because Python 3.10 has reached end-of-life. The minimum `langchain-core` requirement moves up to 1.8.0. Developers who stay on Python 3.10 will keep resolving the 1.4.x line rather than moving to this release. Alongside the version change, the update adds model router and auto mode middleware, labels `ModelRouterMiddleware` as beta, and adds documentation covering the minimum Baseten version. The release is available now to Python developers installing from the project's published packages.