leftover references to Elasticsearch

[rursprung]

there are some leftover references to elasticsearch in the code (besides the copyright notices & co. which have to stay).

the following regex can find those (with only 1-2 false positives, while filtering out the copyright related ones): Elasticsearch(?!.*B.V.)(?! licenses)(?! under one or more) (note: this doesn't work on GitHub, but it e.g. works in IntelliJ)

these should probably be replaced with references to OpenSearch.

[dblock]

Yes, please.