Regression process docs outdated

I was looking for our latest regression process so I started in https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md. The regressions heading links to https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/contributing/issue_workflow.md#regression-issues which mentions we still use regression issues.

However, @DouweM noted that we no longer do this and the docs look outdated. The current process is documented at https://gitlab.com/gitlab-org/gitlab-ce/blob/master/PROCESS.md#regressions

We should consolidate and make sure all docs are updated with the proper procedures.