Upgrade Elasticsearch version in CI & docker to 7.14.2

We have upgraded the production environment to 7.15.1 (gitlab-com/gl-infra/production#5758 (closed)) so we want to use the same version in CI and the Dockerfile for the indexer. Dockerhub does not have version 7.15.1 available yet, so we will have to use the latest 7.14.2. A follow up issue has been opened #92 (closed) to upgrade the indexer once that version becomes available.