Upgrade the version of Elasticsearch used in GDK to 7.15.1

Overview

We are planning on upgrading our production environment to 7.15.1 (gitlab-com/gl-infra/production#5758 (closed)) so we want to use the same version in GDK.

Proposal (optional)

Update the version of Elasticsearch used in this config file: https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/lib/gdk/config.rb#L405