fix(cluster)!: rename X-Upkeep-Secret header to X-Uptop-Secret

Last upkeep-era name in the wire protocol. Breaking for mixed-version
clusters, but zero installed base exists pre-v0.1.0 — free now, breaking
forever after first tag.

internal/cluster/cluster_test.go

@@ -113,7 +113,7 @@ func TestFollowerLoop_SendsSecret(t *testing.T) {
 	var receivedSecret string
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		mu.Lock()
-		receivedSecret = r.Header.Get("X-Upkeep-Secret")
+		receivedSecret = r.Header.Get("X-Uptop-Secret")
 		mu.Unlock()
 		w.WriteHeader(200)
 		w.Write([]byte("OK"))