Onchain scraper in `dispute-coordinator` will scrape `SCRAPED_FINALIZED_BLOCKS_COUNT` blocks before finality (#7013)

* Onchain scraper in `dispute-coordinator` will scrape `SCRAPED_FINALIZED_BLOCKS_COUNT` blocks before finality

The purpose is to make the availability of a `CandidateReceipt` for finalized candidates more likely.

For details see:  https://github.com/paritytech/polkadot/issues/7009

* Fix off by one error

* Replace `SCRAPED_FINALIZED_BLOCKS_COUNT` with `DISPUTE_CANDIDATE_LIFETIME_AFTER_FINALIZATION`