Update dependency solidity-coverage to v0.7.21

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
solidity-coverage	0.7.20 -> 0.7.21

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Release Notes

sc-forks/solidity-coverage

### [`v0.7.21`](https://togithub.com/sc-forks/solidity-coverage/blob/HEAD/CHANGELOG.md#​0721--2022-04-24) [Compare Source](https://togithub.com/sc-forks/solidity-coverage/compare/6b0dab0ff57720943acc76bc0c6528d888c28d3b...d248caae3cd4dbbd160bdfc6ce8a7b03256d807a) \=================== - Add support for UncheckedStatement blocks ([https://github.com/sc-forks/solidity-coverage/issues/712](https://togithub.com/sc-forks/solidity-coverage/issues/712)) - Lazy load hardhat plugin resources ([https://github.com/sc-forks/solidity-coverage/issues/711](https://togithub.com/sc-forks/solidity-coverage/issues/711))

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