Remove the "plugin_id"/"settings" sub-level of index plugin settings

This should implement it, and at least the PhpUnit tests pass for me, locally.
The only non-trivial change is this place in \Drupal\search_api\Plugin\search_api\datasource\ContentEntity::getIndexesForEntity():

    $index_names = \Drupal::entityQuery('search_api_index')
      ->condition('datasource_settings.*.plugin_id', $datasource_id)
      ->execute();

Without the extra plugin_id key, I don't think we can do this anymore. Or is there some entity query whiz who knows a solution?
Like it is, I think we'll have to load all indexes and check them manually. (Not that this should matter much, I'd expect very few sites would have more than one or two indexes.)

Wanted to quickly get this in before, but now we need an upgrade path.

Re-roll.

Not sure how this happened, but this should fix it.

Ah, finally green again!
Thanks also for your review, Joris!
Committed.