We can use this to track which stacks have been created using the library, and with which version.

The absence of this tag suggests the stack hasn't been created with the cdk library.

Mocking this means we can make the value static and it doesn't need updating when new versions of the library are published.

Configuring Jest's `setupFilesAfterEnv` means we apply the mock globally. See https://jestjs.io/docs/en/configuration.html#setupfilesafterenv-array.