prometheus: upgrade alertmanager to v0.31.0

Author: Firefishy

cookbooks/prometheus/recipes/server.rb

@@ -71,7 +71,7 @@
 cache_dir = Chef::Config[:file_cache_path]
 
 prometheus_version = "3.9.1"
-alertmanager_version = "0.30.0"
+alertmanager_version = "0.31.0"
 karma_version = "0.122"
 
 directory "/opt/prometheus-server" do