Remove panels support

Problem/Motivation

This module started out with panels as the intended display mechanism to place blocks in layouts. But as of March 2022 there is no further development on the panels module. And use of Layout Builder is encouraged in all Drupal 8+ installations.
There currently still is D9 and D10 compatibility, so the paragraph blocks 3.x branch can still support Panels while that lasts. But the 4.x branch should move away from panels support and encourage the use of layout builder too.

Proposed resolution

The code should be removed in an upcoming major release.

Remaining tasks

• Mark the code as deprecated in 3.x.
• Make a new major release dropping support for panels.
• Update the module's readme, .info.yml and hook_help implementation.
• Update the project page.