Update gitlab-org/build/omnibus-mirror/alertmanager from 0.25.0 to 0.26.0

Changelog: changed

Author: dependencies-bot

config/software/alertmanager.rb

@@ -20,7 +20,7 @@
 require "#{Omnibus::Config.project_root}/lib/gitlab/prometheus_helper"
 
 name 'alertmanager'
-version = Gitlab::Version.new('alertmanager', '0.25.0')
+version = Gitlab::Version.new('alertmanager', '0.26.0')
 default_version version.print
 
 license 'APACHE-2.0'