Enable indexing to OpenSearch v2.16

[marians]

Towards https://github.com/giantswarm/giantswarm/issues/26502

This PR updates our indexing code, which so far was used against ElasticSearch v6.8, to use OpenSearch v2.16. The goal is to replace our very old ElasticSearch with a more powerful and up-to-date component, to get rid of some problems with the old ElasticSearch.

This includes these changes:

• The need to authenticate for API access with basic authentication (username, password)
• It is no longer possible/necessary to specify a document type when indexing, as each index only supports one document type.
• We are switching to the opensearch-py client, owned by the same project/community that maintains OpenSearch itself.