cmd/hello: migrate to hello.ts.net as the hostname

But still support hello.ipn.dev for a bit.

Updates tailscale/corp#1327

Change-Id: Iab59cca0b260d69858af16f4e42677e54f9fe54a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f