Feedback Button Block can be converted to Premium Content Block, but that seems unintentional

I am able to convert the the `Feedback Button block` to `Premium Content Block`. That does not seem expected because this is not a functionality that visitors would like to offer as a premium package. 🤔

### Purely additional notes

In fact, after doing so, I am not able to add a new `Feedback Button Block`. That block is greyed out.

But that's not entirely related to converting to `Premium Content Block`. It seems that the `Feedback Button block` is limited to one per page.

Start in the crowdsignal-forms repository by reproducing the conversion from Feedback Button block to Premium Content Block in the Gutenberg editor. Check the block registration and conversion behavior, then verify that Feedback Button cannot be converted to Premium Content and that its one-per-page behavior remains intentional and consistent.

Written by the indexing model from the issue text.