Restore IndicesNamespace::deleteMapping() to keep backwards compatibility

[cweiske]

With 6.8.0, this method got removed, which broke the semantic
versioning contract.

This commit restores the method so that applications using that method do
not break.

See https://www.drupal.org/project/elasticsearch_connector/issues/2824539

Resolves: #1235

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[cla-checker-service]

❌ Author of the following commits did not sign a Contributor Agreement:
8c11823

Please, read and sign the above mentioned agreement if you want to contribute to this project

[cweiske]

I will not sign a CLA.