Block Delimiter / Scanner: retire in favor of Core's upcoming WP_Block_Processor

We currently have a `Block_Scanner` and a `Block_Delimiter` class available in the Block delimiter package:
https://github.com/Automattic/jetpack/tree/trunk/projects/packages/block-delimiter

Since WordPress 6.9 will introduce a `WP_Block_Processor` class, we could switch to using it, and retire our package, **once Jetpack requires WordPress 6.9**.

Reference: https://core.trac.wordpress.org/ticket/61401

Start by reviewing the projects/packages/block-delimiter package and the WordPress 6.9 requirement for Jetpack. Trace where Block_Scanner and Block_Delimiter are used, then verify that the package can be retired and its uses switched to Core's WP_Block_Processor once the minimum WordPress version permits it.

Written by the indexing model from the issue text.