Bump Yams version.

⚠️ Technically this release performs a major version bump of the Yams library.

Looking over the Yams v3 release, I am inclined to say that v3 of Yams is fully source-compatible with v2 of Yams. There is a noted breaking change of now requiring Swift 4.1+ but OpenAPIKit only supports Swift 5.1+ to begin with.

The upshot of this major version bump is that even though OpenAPIKit only uses Yams for test targets, now downstream packages will not experience dependency conflicts when using OpenAPIKit and trying to use the latest versions of Yams (which include some important bug fixes).