Align starlette to 0.28.0 in all subpackages
This PR aligns the `starlette` version to 0.28.0 in all subpackages.

Note that before this PR:
- in the `admin` and `worker` services, `starlette` was pinned to 0.27.0
- whereas in the `libapi`, `starlette` was pinned to 0.28.0

Thus, the `admin` and `worker` services could not use the `libapi` (subsequent PR).
Align starlette to 0.28.0 in all subpackages: This PR aligns the `starlette` version to 0.28.0 in all subpackages.

Note that before this PR:
- in the `admin` and `worker` services, `starlette` was pinned to 0.27.0
- whereas in the `libapi`, `starlette` was pinned to 0.28.0

Thus, the `admin` and `worker` services could not use the `libapi` (subsequent PR).