Chore: Use `GsonFactory` as `JsonFactory` implementation

[Capstan]

For a variety of reasons, mostly to do with historical security issues, the monorepo and Google security prefers Gson to Jackson, and thus GsonFactory to JacksonFactory. Pushing this change upstream in lieu of patching the monorepo import.

[meredithslota]

(Related internal issue for maintainers b/216657461 for a different library, for additional context.)