Adjust group API, scenario and web group thresholds

Nailia Iskhakovarequested to merge
adjust-ttfb-threshodls-api-web into main

Adjusted group API, scenario and web group thresholds. Added notes in relevant performance issues gitlab-org/gitlab#386475 (comment 2469587391) and gitlab-org/gitlab#364824 (comment 2469591462),

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