Using searchfox.org, the correct preference name seems to be browser.search.serpEventTelemetryCategorization.enabled, as I couldn't find any reference to serpEventTelemetry.enabled
https://searchfox.org/mozilla-central/search?q=serpEventTelemetry&path=&case=false®exp=false
This change updates browser.search.serpEventTelemetry.enabled to browser.search.serpEventTelemetryCategorization.enabled in the default LibreWolf preferences.