Starlette is imported in the API but not pinned in requirements.txt

[jonnyry]

Minor observation following PR #5032

The API imports Starlette in main.py:

AzureTRE/api_app/main.py

Lines 11 to 12 in 5cf89f0

	from starlette.exceptions import HTTPException
	from starlette.middleware.errors import ServerErrorMiddleware

But it is not tracked or pinned in the API's requirements.txt