Un-skip migration batch size test

[mshustov]

I had to skip a SO migration integration test
b88c94c
due to a bug in Elasticsearch elastic/elasticsearch#79684

Un-skip the test when either the bug is fixed in Elasticsearch or elasticsearch-js implements a workaround.

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[rudolf]

Also affects the currently skipped test "resolves left request_entity_too_large_exception when the payload is too large" https://github.com/elastic/kibana/blob/master/src/core/server/saved_objects/migrationsv2/actions/integration_tests/actions.test.ts#L1542