Remove ParagraphsCallback and require Drupal core >=9.2

Problem/Motivation

ParagraphsCallback was added for backporting the extended functionality of Drupal core 9.2+ Callback migrate process plugin to older Druapl core versions. But since core 9.2.x is the latest supported core version, AND since we are still in aplha stability, this is the best opportunity to get rid of the backport.

Remaining tasks

1. Remove ParagraphsCallback class
2. Remove the appropriate lines from the process plugin alter hook
3. In the info file, change the value of core_version_requirement from ^9 to ^9.2

API changes

Newer core version required.