Update node version for gatsby-source-ghost

Update node version to support node 20.

We’re getting some warnings about the node version in our current SPA with Gatsby.

I’ve opened an issue and PR in the repo added support for node 20 by chrism1996 · Pull Request #277 · TryGhost/gatsby-source-ghost (github.com)

Node v20 works fine with Ghost, could anyone update the dependency? Thanks