• https://fastapi.tiangolo.com/deployment/versions/#about-starlette

You shouldn't pin the version of starlette.
Different versions of FastAPI will use a specific newer version of Starlette.
So, you can just let FastAPI use the correct Starlette version.

• https://pip.pypa.io/en/stable/user_guide/#requirements-files

Requirements files are used to hold the result from pip freeze for the purpose of achieving Repeatable Installs.