[lib] Replace LegacyRawThreadInfoS with RawThreadInfoS where simple

Summary:
Ugh, totally thought I'd handled ripping out most of the instances of LegacyRawThreadInfo... but remembered I need to handle LegacyRawThreadInfoS (plural) after running into some new Flow issues.

Will try to get through these ASAP.

Depends on D10223

Test Plan: CI/flow/etc.

Reviewers: ashoat, ginsu, tomek, rohan

Reviewed By: rohan

Differential Revision: https://phab.comm.dev/D10224