Ignore PHPUnit in depdendabot config until #40 is actioned

> You can... ignore all major, minor, or patch releases for a dependency by adding an [`ignore` condition](https://docs.github.com/en/code-security/supply-chain-security/configuration-options-for-dependency-updates#ignore) with the desired `update_types` to your config file.

_Originally posted by @dependabot[bot] in https://github.com/bigbite/phpcs-config/issues/48#issuecomment-1200857856_