deprecate ATT&CK course-of-action and replace by new mitigations

[cvandeplas]

See https://github.com/mitre/cti/pull/65#issuecomment-546356114

Prior to the July 2019 ATT&CK update (also known as ATT&CK-v5.0) mitigation objects in the Enterprise domain had 1:1 relationships with techniques, and were kept on the technique pages. Thus the external reference URL pointing to a technique page, and the ID which implies that it is a technique (even though from a STIX perspective it is not). You can see this behavior in action via our previous versions archive here. In the July 2019 update referenced above, we refactored our enterprise mitigations to support many:many relationships with techniques. This update involved deprecating all of the old enterprise course-of-action objects in favor of the new mitigations. Deprecated objects (marked with the x_mitre_deprecated field) are no longer supported by ATT&CK. They are kept in our repo for the purposes of historical record and to avoid breaking any code that may rely on those specific objects. However, their external references URLs are not guaranteed to lead to live pages on our website, and we do not provide updates to their content. In the case of the mitigations such as T1168 they were removed from the website altogether.

And also : https://attack.mitre.org/mitigations/enterprise/