joint-discovery-gatk4-local.wdl uses deprecated output syntax

In the GitHub-provided, official WDL files for running GATK, the "local" version of joint discovery uses a deprecated form of declaring outputs, which prevents it from being run as a subworkflow.

https://github.com/gatk-workflows/gatk4-germline-snps-indels/blob/master/joint-discovery-gatk4-local.wdl

Note that another user already has a pull request with a fix for this issue. However, that has been pending for nearly a month now, so might be worth accepting:

https://github.com/gatk-workflows/gatk4-germline-snps-indels/pull/29