Drop dependency on github.com/cyberdelia/go-metrics-graphite

The go-metrics-graphite package has been unmaintained for 10+ years,
which is a packaging and supply-chain concern for downstream
distributors (see #1831). Nebula only used its Config struct and Once()
entrypoint, so inline just those (~65 lines) into a local graphite.go,
preserving the upstream BSD-2-Clause copyright notice, and remove the
dependency.

Fixes #1831

Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>

stats_test.go

@@ -371,7 +371,7 @@ func waitForListening(t *testing.T, addr string) {
 	})
 }
 
-// graphiteSink is a minimal TCP accept-and-discard server so graphite.Once
+// graphiteSink is a minimal TCP accept-and-discard server so graphiteOnce
 // calls in tests don't spam error logs or wedge on connection refused.
 type graphiteSink struct {
 	ln net.Listener