Update docker.io/metio/matrix-alertmanager-receiver Docker tag to v2025.5.21

roles/custom/matrix-alertmanager-receiver/defaults/main.yml

@@ -11,7 +11,7 @@
 matrix_alertmanager_receiver_enabled: true
 
 # renovate: datasource=docker depName=docker.io/metio/matrix-alertmanager-receiver
-matrix_alertmanager_receiver_version: 2025.4.23
+matrix_alertmanager_receiver_version: 2025.5.21
 
 matrix_alertmanager_receiver_scheme: https