## What changes were proposed in this pull request?

Go back to selecting source/target 1.7 for Scala 2.10 builds, because the SBT-based build for 2.10 won't work otherwise.

## How was this patch tested?

Existing tests, but, we need to verify this vs what the SBT build would exactly run on Jenkins

Author: Sean Owen <sowen@cloudera.com>

Closes #16983 from srowen/SPARK-19550.3.