[examples/blog-starter-typescript] Upgrade to TypeScript v4 (#18225)
[examples/blog-starter-typescript] Upgrade to TypeScript v4 (#18225)

Co-authored-by: Luis Alvarez D <faea5242a00c52da62a0f00df168c199b7ab748d@vercel.com>
zeit/next.js,flybayer/next.js,azukaru/next.js,zeit/next.js,flybayer/next.js,JeromeFitz/next.js,azukaru/next.js,zeit/next.js,flybayer/next.js,JeromeFitz/next.js,flybayer/next.js,azukaru/next.js,JeromeFitz/next.js,JeromeFitz/next.js,azukaru/next.js