core vitals: update CLS to new definition

[vigneshshanmugam]

• Update CLS to the new variant - https://web.dev/evolving-cls/ which uses the windowed approach of max-session-gap1s-limit5s .

• As the UI does not utilize the previous CLS values, we will go ahead with not suporting the previous value which only calculated the CLS for main frames and instead report only the windowed CLS value for all frames under cumulativeLayoutshift name.