[Whitelisted Caller] Update schedulerParams.lookahead configuration parameter to 3

This referenda increases schedulerParams.lookahead value to 3.
Why this change is needed?

scheduler.lookahead determines the size of the claim queue and at the moment is set to 2. The claim queue shows which parachains are about to be scheduled on a specific core. A value of two shows what will be scheduled for a window of two blocks.

asyncBackingParams.allowedAncestryLen is an async backing parameter which determines the oldest relay parent a candidate can be built on. 2 here means that a candidate can be built on the latest know relay parent or the two relay parents before it. In total this gives a window of three blocks.

So we have got a discrepancy. Async backing will accept candidates based on three different blocks while the claim queue shows information only about two. Bumping schedulerParams.lookahead to 3 will correctly align these values.

The same change was executed on Kusama: https://kusama.subsquare.io/referenda/488

Link to the whitelisting referenda on Collectives: https://collectives.subsquare.io/fellowship/referenda/288