migrations: `take()`should consume read and write operation weight (#4302)

#### Problem
`take()` consumes only 1 read worth of weight in
`single-block-migrations` example, while `take()`
[is](https://github.com/paritytech/polkadot-sdk/blob/master/substrate/frame/support/src/storage/unhashed.rs#L63)
`get() + kill()`, i.e should be 1 read + 1 write. I think this could
mislead developers who follow this example to write their migrations

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Co-authored-by: Bastian Köcher <[email protected]>