Use EntityTypeManagerInterface instead of EntityManagerInterface in TermMachineName Views argument validator

Problem/Motivation

I am not entirely sure when it became a requirement to switch from EntityManagerInterface to EntityTypeManagerInterface, but I am developing in 8.7-beta2 and there is a fatal error being triggered at the construction of the TermMachineName class (namespace Drupal\taxonomy_machine_name\Plugin\views\argument_validator).

The deprecation notice for EntityManagerInterface says it will be removed "before 9.0", and I can't find any change records requiring it to be replaced with EntityTypeManagerInterface in 8.7. I do believe that EntityTypeManagerInterface has been useable since 8.1. So I don't think there is any downside to implementing this change now.

Steps to reproduce

1. In a new or existing view, click to add a contextual filter.
2. Choose a field. I don't think it matter which you choose.
2. You won't reach the configuration dialog since that AJAX load will fail when attempting to construct the plugin.

Watchdog shows:

TypeError: Argument 4 passed to Drupal\taxonomy_machine_name\Plugin\views\argument_validator\TermMachineName::__construct() must implement interface Drupal\Core\Entity\EntityManagerInterface, instance of Drupal\Core\Entity\EntityTypeManager given