Backend: Add a 2000 limit to the number of jobs that a pipeline can have

Problem

Users are creating really large pipelines with over 1000 jobs and GitLab does not handle these jobs very well.

Proposed Solution

Set a limit of 2000 jobs in create pipeline service https://gitlab.com/gitlab-org/gitlab/-/blob/a91773e2ee15d550ffa363836ffd8b630965e54d/app/services/ci/create_pipeline_service.rb#L21 and when attempting to be exceeded, it will fail which would return a limit exceeded error so that the user can be informed of why the pipeline failed.

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