Legacy index (pre-7.0) not able to ingest data after upgrade to 7.6.2

After upgrading a cluster with legacy indexes (i.e. which uses gocouchbase for data ingestion), they aren't able to ingest data due to a recent change introduced in 7.6.2. This is due to the fact at https://github.com/couchbase/cbft/blob/f81ec8920f9b022f7d605feee1907eb4c664379a/pindex_bleve.go#L2374 we're doing a gocbcore specific check without keeping in mind of other feed types. So when the legacy index tries to ingest data, the path that hits the above line is via gocouchbase feed and that causes an error (as seen in CBSE-17796).