fix: More explicit peerDep on gatsby

[LekoArts]

Hello @hupe1980, Gatsby maintainer here 👋
First and foremost, thanks for creating the plugin & maintaining it. Much appreciated!

While looking at your plugin I noticed that the peerDependency on gatsby is set incorrectly. We're in the process of providing more helpful information on the /plugins page of our website and for that we need plugins to set their peerDependencies correctly/more specific.

At the moment you say that any version (v4, v5, v6, etc.) will work with this even when in between we'd introduce breaking changes.

Making it more explicit is the safer choice (in the future you'll be able to mark breaking changes with explicit version ranges) and will allow us to display the plugin as compatible to versions X, Y, Z.

Thanks!

[hupe1980]

Okay, thx for the hint!