build: Fix setting the incorrect bwc version in mixed cluster qa module
build: Fix setting the incorrect bwc version in mixed cluster qa module

Prior to this change if the `bwcTest` task is run then it would create
task for each version, but each task in reality would use wireCompatVersions - 1
ES version. So we were not actually testing against 5.6.x versions in the
6.x and 6.0 branches.

strapdata/elassandra,strapdata/elassandra,vroyer/elasticassandra,vroyer/elasticassandra,vroyer/elasticassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra,vroyer/elassandra