Error on too-low `// Minimum TypeScript Version:`

[sandersn]

Authors should be prompted to remove too-low // Minimum TypeScript Version: comments. For example:

// ...
// Minimum TypeScript Version: 2.8

Is still quite common, and misleading, since the file may now contain 3.0- and 3.1-only features.

Adding this will require a cleanup run to remove all the existing outdated comments.