PCH: update error message on AI-powered excerpt generator

[danielabloch]

Is your feature request related to a problem?

When users attempt to use the AI-powered excerpt generator, but do not have access to it, they see this message:

Related to the snackbar updates we want to make in the Tools tab of the PCH: https://github.com/Parsely/wp-parsely/issues/2424

Describe the solution you'd like

Repeat the same message we show in the Tools tab of the PCH to customers who do not yet have access to the AI features.

Describe alternatives you've considered

None, because we decided on and implemented this message for other AI powered features. With this request we ensure the same message shows for all features.

Additional context