Primary partitions of single partitioned indexes with replicas are not evenly distributed

Here's the scenario:

1. Create a 2 search node cluster
2. Create a bucket
3. Set up 2 single partitioned indexes with replica=1 
4. Each node should contain 1 primary and 1 replica, however it seems both primaries reside on the same node.

This has already been addressed in 7.5.0 builds with https://review.couchbase.org/c/cbgt/+/181231 . This ticket is to track the backport of this fix.