Prometheus alert query is created incorrectly

It seems as though since !31362 (merged) the variable substitution has been broken when creating Prometheus Alerts through the GitLab UI.

Running Clusters::ApplicationsPrometheusConfigService doesn't seem to substitute the params anymore.

Cause:

Previously we were using String#% to replace the strings which expects the input string to contain %{var_to_replace}. As this format was removed in the MR we encounter the issue.