Use non-deprecated appengine maven plugin for deploying portfolio

[andreahu12]

copied from https://github.com/google/software-product-sprint/pull/50

I might be wrong because I haven't set up billing for my experimental project (and therefore haven't deployed successfully), but following the instructions and trying to deploy with mvn appengine:update was failing with "Deployments using appcfg are no longer supported. See https://cloud.google.com/appengine/docs/deprecations"

To get further in the deployment process, I think it needs to use a non-deprecated plugin to deploy. Using mvn appengine:devserver only works for locally testing with the old plugin. mvn appengine:run can be used to test locally for both.

Current plugin (deprecated): https://github.com/GoogleCloudPlatform/gcloud-maven-plugin New plugin (not-deprecated): https://github.com/GoogleCloudPlatform/app-maven-plugin

[apicardgoog]

Thank you for making this change here too.