Mark active pip for the telemetry level slider with a more contrastful color

[iOvergaard]

It seems it is also possible to interact with pips, so maybe we should highlight the active pip? https://refreshless.com/nouislider/examples/#section-pips

Discovered in: 10.1.0-rc2

Originally posted by @bjarnef in https://github.com/umbraco/Umbraco-CMS/issues/12771#issuecomment-1202484400