Paywall block: switch to standard block editor type check

### Impacted plugin

Jetpack

### What

Refactor block's unregistration to backend.

### How

In Paywall block we check for post type to decide if to _unregister_ the block after it has already been registered(added in https://github.com/Automattic/jetpack/pull/32190). The block should not be available in templates, pages, etc.

https://github.com/Automattic/jetpack/blob/a59af2aba8d972b14fe87e8a858988474c4c074b/projects/plugins/jetpack/extensions/blocks/paywall/editor.js#L6-L18

Once loading blocks via `block.json` is supported, we can switch to more standards way of not registering a block based on if the post type is `post`, and do it in PHP instead, see:

https://developer.wordpress.org/reference/hooks/allowed_block_types_all/#comment-5624
https://developer.wordpress.org/block-editor/reference-guides/filters/block-filters/#allowed_block_types_all

Original slack convo p1690892655610999-slack-C052XEUUBL4

Start with projects/plugins/jetpack/extensions/blocks/paywall/editor.js and the documented allowed_block_types_all filter references to understand the current client-side unregistration and the intended backend entry point. Done means the Paywall block is available for posts but not templates, pages, or other post types, with the relevant checks passing.

Written by the indexing model from the issue text.