fix filter syntax for dlq alert

[k4leung4]

│ Error: Error creating AlertPolicy: googleapi: Error 400: Field alert_policy.conditions[0].condition_threshold.filter had an invalid value of "        metric.type="pubsub.googleapis.com/topic/send_request_count"
│         resource.type="pubsub_topic"
│         metadata.system_labels."name"=~".*-dlq-.*"
│         metadata.system_labels."name"!=monitoring.regex.full_match(".*ing-vuln-dlq.*")
│ ": Could not parse filter "        metric.type=\"pubsub.googleapis.com/topic/send_request_count\"\n        resource.type=\"pubsub_topic\"\n        metadata.system_labels.\"name\"=~\".*-dlq-.*\"\n        metadata.system_labels.\"name\"!=monitoring.regex.full_match(\".*ing-vuln-dlq.*\")\n"; syntax error at line 3, column 38, token '=~'
│ 
│   with module.global-alert.google_monitoring_alert_policy.pubsub_dead_letter_queue_messages,
│   on .terraform/modules/global-alert/modules/alerting/main.tf line 369, in resource "google_monitoring_alert_policy" "pubsub_dead_letter_queue_messages":
│  369: resource "google_monitoring_alert_policy" "pubsub_dead_letter_queue_messages" {