Docs still show outdated type casts on aggregation functions

[fnimick]

After drizzle-team/drizzle-orm#1487 - these type casts are no longer necessary, and users should be guided to the new aggregation functions directly rather than explaining how the no-longer-necessary type casts work.

https://orm.drizzle.team/docs/select#aggregations

(yes, the function on aggregation helpers appears afterward, but this is only after the discussion on type casts from the sql count, etc functions. Ideally we could find a better example for type casting or .mapWith() than one which should never be used anymore.)