Stop recommending Mockingbird for test mocks

[bbrk24]

Checklist

• I searched the existing GitHub issues and this hasn't already been reported.

Request

Mockingbird may have been a good choice of library two years ago when Dependiject was first created, but it hasn't been updated since then. More pressingly, Mockingbird's generated mocks don't compile in Swift 6 mode -- so even if #82 adds support for Dependiject itself compiling in Swift 6 mode, consuming projects that use Mockingbird will be stuck in Swift 5 mode.