Fixes initial deployment step

What does this MR do?

Changes initial deployment step from gprd-cny-migrations to gprd-migrations, since deployment notifications are sent only on gprd

From https://ops.gitlab.net/gitlab-org/release/tools/-/jobs/2601733, we can see the initial job is gprd-migration instead of gprd-cny-migrations

2020-12-14 16:27:38.382339 I ReleaseTools::Tasks::AutoDeploy::CheckProduction -- Checking production deployment status -- {:deploy_version=>"13.7.202012141050-b9dd263b905.3b6a0dc15c3", :deployment_step=>"gprd-migrations 50%", :deployment_job_url=>"https://ops.gitlab.net/gitlab-com/gl-infra/deployer/-/jobs/2601754"}