BC break in 1.8

Hi there,

The commit https://github.com/cebe/php-openapi/commit/219813f7e2826d782e6b705347dac654719b21ba introduces a backwards incompatible change to the security property of the OpenApi an Operation Classes. This is an issue, as the major version of the package stayed the same, which causes issues in packages which depend on this library, for example https://github.com/thephpleague/openapi-psr7-validator/issues/248. Also, the property documentation was not updated in accordance with the new change. I don't know if it's worth the trouble to reverse the change and reintroduce the change in a version 2 of this package, but you should be aware about the issues that were caused by this change.