fix: update `@imgix/gatsby` to support Gatsby v5.10.0

[angeloashmore]

Package

• [x] gatsby-source-prismic
• [x] gatsby-plugin-prismic-previews

Types of changes

• [ ] Chore (a non-breaking change which is related to package maintenance)
• [x] Bug fix (a non-breaking change which fixes an issue)
• [ ] New feature (a non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Description

This PR updates @imgix/gatsby to the latest version. Updating the package fixes a bug preventing Gatsby >= v5.10.0 projects from starting or building.

See #531 for more details.

Checklist:

• [ ] My change requires an update to the official documentation.
• [x] All TSDoc comments are up-to-date and new ones have been added where necessary.
• [ ] All new and existing tests are passing.