Enable feature h3 in reqwest 0.12.5 adds breaking change: `Sync` implementation on return future is removed

[NobodyXu]

With reqwest 0.12.5, if I enable the feature h3, then the Sync implementation on the returned future is removed.

While I know that h3 is an unstable feature, I think this is.a breaking change since adding new feature should not change return type (bound) of existing function, and the removal of Sync is a bit surprising given that it works without h3.