Deprecate calling Drupal\block\Plugin\migrate\process\BlockTheme::_construct() with the $migration argument

Problem/Motivation

In #3319582: Fix calls to methods with too many parameters passed in we discovered the $migration argument in the constructor of Drupal\block\Plugin\migrate\process\BlockTheme is unused.

Let's deprecate it in 10.1 and remove it in 11.0.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet