Clean leftovers of old ElasticSearchIO versions / test mechanism

While auditing about flakiness issues, I saw lefovers on the test code and production code of the IO. This list is not comprehensive but spots some places:

• In ESTestCommon#testSplit there is a reference to ES2: ES2 cannot split the shards (missing API) so the split are fixed to the number of shards (5 by default in the test). As ES2 support was removed, this special case in the test can be removed
• org.elasticsearch.bootstrap.JarHell was a hacky fix to disable jarHell test in ES test framework. This framework is no more used (testContainers) so we could remove the hack in all places
• if (backendVersion == 2)  in prod code + remove shardPreference from BoundedElasticsearchSource
• @ThreadLeakScope(ThreadLeakScope.Scope.NONE) come from es test framework that is no longer used
• remove module elasticsearch-tests-2
• please audit for other places that could need cleaning.