8.0.0 - One up

BC breaks

• raised minimum required PHP version to 7.3
• stubbles\input\broker\RequestBroker::procure() doesn't return the given and procured object instance any more
• stubbles\input\filter\range\StringLength doesn't support instances of stubbles\values\Secret any more
• stubbles\input\ValueReader::asSecret() doesn't take a stubbles\input\filter\range\StringLength any more but stubbles\input\filter\range\SecretMinLength
• @Request[Secret] doesn't support the maxLength attribute any more
• @Request[OneOf] will throw an exception when callback specified with allowedSource doesn't exist or isn't callable