Widen dependencies to allow @google-cloud/pubsub ^2.0.0

[robhogan]

Thanks for the package :)

None of the breaking changes introduced in @google-cloud/pubsub 2.0.0 have any impact on this package, so it's safe to widen this constraint.

This change allows consumers to use this package with the latest google libs.

[tubbo]

+1 @jonas-app @TimSusa i think this will fix node-forge dependency warnings in dependabot/renovate alerts because the newest version of @google-cloud/pubsub has this dependency updated.