Fix backwards-incompatible Json data_parser API change included in 6.0.3.

NW for some minor feedback.

Applied suggestions from @lucashedding and a couple of other changes. I think this is ready for review again.

Nice enhancements!

@joegraduate, @heddn, Thank you! 🙏

FYI, that change broke all my migrations... After I reverted to 6.0.2, all my migrations ran successfully again...
Not really sure what information I can provide, somehow it seems no rows was detected by the migration.

https://www.drupal.org/project/migrate_plus/releases/6.0.4 → does include the BC fix now too.

After upgrading to 6.0.4, the rows count is 0 for all my migrations, reverting to 6.0.2 fixes the issue...

@jsacksick, Could you please check whether the 3465782-using-slash-json-parser branch (MR 102) in 🐛 Using "/" item_selector in Json parser plugin config does not work as expected anymore Needs review fixes the issue you have?

6.0.4 does not fix this issue. Our custom plugin which extends Drupal\migrate_plus\Plugin\migrate_plus\data_parser\Json::getSourceData is still broken:

PHP Fatal error: Declaration of Drupal\CustomJson\Plugin\migrate_plus\data_parser\CustomJson::getSourceData(string $url): array must be compatible with Drupal\migrate_plus\Plugin\migrate_plus\data_parser\Json::getSourceData(string $url, string|int $item_selector = '')

Automatically closed - issue fixed for 2 weeks with no activity.