The hybrid search project added a new internal aggregate IDL command option, $_isRankFusion, that is used when mongos sends $rankFusion queries to the shards. 

This field was renamed to $_isHybridSearch in SERVER-104725, committed in 8.2. Because of this, when we upgrade a sharded cluster, a not-yet-upgraded mongos may send $_isRankFusion to upgraded shards and the shards will return an error due to not recognizing the field, failing the query.

We should port the rename of this field to 8.0, so that we won't run into this issue on 8.0 -> 8.2 upgrades.

Note: this backport is only safe (i.e., won't cause issues on 8.0 upgrades) because we have not yet released $rankFusion on 8.0, so users will not be running affected queries during the upgrade.