Deprecate Pipeline API `POST /projects/:id/pipeline` (Create a new pipeline)

Description

There are two API for creating a pipeline.

• https://docs.gitlab.com/ce/api/pipelines.html#create-a-new-pipeline (POST /projects/:id/pipeline)
• https://docs.gitlab.com/ce/ci/triggers/README.html#trigger-a-pipeline (POST /projects/:id/trigger/pipeline)

pipeline was meant to override trigger/pipeline (See https://gitlab.com/gitlab-org/gitlab-ce/issues/23468), however, recently there was an update for trigger/pipeline in gitlab-ce%"9.0" and trigger/pipeline is more flexible now. Therefore I thought to deprecate POST /projects/:id/pipeline does make sense.

Proposal

Deprecate POST /projects/:id/pipeline endpoint

Links / references

/cc @grzesiek @zj @ayufan