Expose skipping branch pipelines project setting

What does this MR do and why?

• General idea from the Epic: Create at most 1 pipeline when in the context o... (gitlab-org#6044)
• Full implementation POC and screenshots: POC: Solve duplicate MR pipelines (!237729 - closed)
• First step: !241086 (merged)
• Second step: This MR
• Third step: Draft: Add QA specs for the prevent duplicate p... (!241642)

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