Resolve starlette deprecation warnings in test outputs

[davem-bis]

Description

I have:

• Changed the webserver exception type of HTTP_422_UNPROCESSABLE_ENTITY to HTTP_422_UNPROCESSABLE_CONTENT.
• Changed the httpx dev requirement to httpx2, as the use of httpx with starlette.testclient is deprecated.

Test Plan

• For the change to 422 HTTP responses, these are covered by tests in tests/web/test_main.py (including but not limited to; test_rename_file_to_existing_directory, test_create_directory_already_exists, test_rename_directory_already_exists_not_empty)
• For the change of httpx to httpx2, this package is only used in the test tests/web/test_main.py::test_cancel, which still passes.

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the DCO