(3P) Block Previews: Update headstart annotation & API

[obenland]

Dependent on:
WordPress/gutenberg#16033
WordPress/gutenberg#16113
WordPress/gutenberg#16801

• Remove images from headstartdata.
• Update the FSE mu-plugin to not include preview images in headstart annotation.
• Regenerate headstart annotation.
• Double-check topic-verticals endpoint to see if preview-related code exists and needs to be removed.

[getdave]

Just noting here that if Templates are not created with an accessible description then the previews are not accessible. Adding a meta field to the Pages on the master site would allow Template authors to describe the Template which would allow us to add a text-based description of the Template in the Template selector UI.

[obenland]

It turns out we'll need preview images a while longer. Closing until the day that we don't.