Investigate unstable performance-test SLAs

[ReToCode]

Context

PR https://github.com/knative/serving/pull/14289 made performance tests work again. There has been some discussion on follow-up improvements. PR https://github.com/knative/serving/pull/14429 added a new SLA to check if the expected rates match the actual rates. Unfortunately, the performance test SLAs were never really stable in various builds, so we should look into that topic separately.

Goals

• Adapt the performance-tests so that results are stable between multiple runs
• Optionally update the SLA values
• Fail the performance-test build if an SLA is breached

Previous discussions see:

• https://github.com/knative/serving/pull/14429#issuecomment-1890975250
• https://github.com/knative/serving/pull/14429#issuecomment-1891523004
• https://github.com/knative/serving/pull/14429#issuecomment-1891989756

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