feat: Replace @regru/prefer-early-return with unicorn/prefer-early-return

Fixes #449 <!--- Provide a general summary of your changes in the Title above --> <!--- Make sure to read the [Contributing Guidelines on Pull Requests](https://github.com/AndreaPontrandolfo/sheriff/blob/master/CONTRIBUTING.md#opening-a-new-pull-request) --> ## Related Issue <!--- This project only accepts pull requests related to open issues with the `approved` label --> <!--- Failure to meet the above basic requirement will see this PR declined immediately --> <!--- Please link to the issue below 👇 --> Closes #449 ## Other <!--- If there is any other detail you want to add, write it here. -->