[v11.3.x] Alerting: Fix remote Alertmanager readiness check path (#95066)

Alerting: Fix remote Alertmanager readiness check path (#95063)

(cherry picked from commit 4f8f82f5f1)

Co-authored-by: Santiago <santiagohernandez.1997@gmail.com>

pkg/services/ngalert/remote/client/alertmanager.go

@@ -71,7 +71,7 @@ func (am *Alertmanager) IsReadyWithBackoff(ctx context.Context) (bool, error) {
 	ctx, cancel := context.WithCancel(ctx)
 	defer cancel()
 
-	readyURL := am.url.JoinPath(am.url.Path, alertmanagerAPIMountPath, alertmanagerReadyPath)
+	readyURL := am.url.JoinPath(alertmanagerAPIMountPath, alertmanagerReadyPath)
 
 	attempt := func() (int, error) {
 		req, err := http.NewRequestWithContext(ctx, http.MethodGet, readyURL.String(), nil)