chore(next): update to use latest version of go-katapult

[ikadix]

Updates the go-katapult dep to v0.2.9 With this comes a number of changes in the next package.

• We no longer need to check that the status code is 2xx as this is now handled in go-katapult and throws an appropriate error.
• go-katapult also throws an exposed ErrNotFound in the next client now.
• Anything marked as optional in the API schema is now exposed as a nullable.Nullable[T] this is handled appropriately.
  • In a lot of scenarios we call .Get() and can ignore the error returned, relying on the empty type that is returned.