migration_lookup process plugin can't use multiple or specific source keys to use for lookup

I couldn't get #40 working. Here's an updated version that works on Drupal 10/11.

This also needs to be an MR at this point.

As per #47 - needs to go to an MR.

Removing

> 1. The ability to specify multiple source keys

from the IS and the title, as this is now possible:

        $lookup_value = array_values($row->getMultiple($this->configuration['source_ids'][$lookup_migration_id]));

Updated IS again.

There's a backwards compatibility & DX problem we haven't spotted.

We add support for this:

 *       source_ids:
 *         users:
 *           key3: author

but remove support for this, without keys:

 *       source_ids:
 *         users:
 *           author

That's a BC break, and it's also not good DX if you want to specify all the lookup keys.

Fixed bug in migrate process plugin.

(no longer needs reroll)

Think next step would be to get a test case showing the issue. Also issue summary needs some love.