* Don't report MIGRATION_NEEDED for 7.x indices
Eventually, we will need to migrate 7.x indices to 8.x before doing a
significant upgrade of Lucene. However, the migrations to 8.x are not
adequately tested: while they will eventually be needed, they are not
currently needed, and may in fact produce bugs.
This change will ensure that the GET _migration/system_feature API
returns NO_MIGRATION_NEEDED in 8.x. We will begin to require
migrations once we start testing for the next major version upgrade.
Mark Vieira
William Brafford
Mary Gouseti
Rene Groeschke
Joe Gallo