cmd/derper: move 204 handler from package main to derphttp

Updates #13038

Change-Id: I28a8284dbe49371cae0e9098205c7c5f17225b40
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f