benchmarks_sep has had threshold checks disabled on release branches (v8.3+) since SERVER-122533. Once SERVER-122630 makes the GenerateAndCheckPerfResults hook release-branch aware, we should re-enable the hook so Product Performance gets back regression detection on release branches.

While doing this, verify that:

• The benchmarks_sep microbenchmark code and GenerateAndCheckPerfResults hook logic on v8.3 are in sync with master. Missing commits would produce spurious failures when the hook tries to compare against a base commit that was never run.
• benchmarks_sep is scheduled on every mainline commit on v8.3. If it is not, runs will fail looking for absent baselines.
• Confirm with alice.doherty@mongodb.com whether benchmarks_sep should also run in v8.3 PR checks.