chore(deps): update docker.io/metio/matrix-alertmanager-receiver docker tag to v2026.4.15

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: 2026.4.1
+matrix_alertmanager_receiver_version: 2026.4.15
 
 matrix_alertmanager_receiver_scheme: https