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

[jeherve]

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