Premium content blocks ought to prevent their login blocks from being deleted.

### Impacted plugin

Jetpack

### Steps to Reproduce

1. Add a premium content block
2. Select the premium-content/login-button block
3. Delete it
4. Notice that users can't login
5. Add a core/button block that links to wordpress.com/log-in?redirect_to=https://example.com/post123
6. Don't notice that it doesn't give access to the premium content

### A clear and concise description of what you expected to happen.

This actually happened in at least two customer tickets, https://github.com/Automattic/wp-calypso/issues/69427 has more detail.

### What actually happened

_No response_

### Browser

_No response_

### Other information

_No response_

### Platform (Simple, Atomic, or both?)

_No response_

### Reproducibility

Consistent

### Severity

Some (< 50%)

### Available workarounds?

Yes, easy to implement

### Workaround details

Remove and readd the premium-content block pasting the content inside, or remove the children of the _Premium Content buttons_ block which will make the original buttons reappear

https://user-images.githubusercontent.com/93301/212153468-9624348e-a396-461f-a4f3-2eb34e32c362.mp4