newrelic: ids are uuids, not integers.

The previous fixtures were not generated from New Relic directly, so
incorrectly implied that `id` was an integer.  See the examples for
`issueId` in their documentation[^1].

[^1]: https://docs.newrelic.com/docs/alerts-applied-intelligence/notifications/message-templates/

zerver/webhooks/newrelic/fixtures/incident_acknowledged_new.json

@@ -1,6 +1,6 @@
 {
     "incident_acknowledge_url": "https://alerts.newrelic.com/accounts/2941966/incidents/1234/acknowledge",
-    "id": 1234,
+    "id": "3576f543-dc3c-4d97-9f16-5c81f35195cb",
     "details": "Violation description test.",
     "alertPolicyNames": ["Test policy name"],
     "condition_name": "Server Down",

zerver/webhooks/newrelic/fixtures/incident_active_new.json

@@ -1,6 +1,6 @@
 {
     "incident_acknowledge_url": "https://alerts.newrelic.com/accounts/2941966/incidents/1234/acknowledge",
-    "id": 1234,
+    "id": "8ceed342-f305-4bfa-adb8-97ba93f5dd26",
     "details": "Violation description test.",
     "alertPolicyNames": ["Test policy name"],
     "condition_name": "Server Down",

zerver/webhooks/newrelic/fixtures/incident_closed_new.json

@@ -1,6 +1,6 @@
 {
     "incident_acknowledge_url": "https://alerts.newrelic.com/accounts/2941966/incidents/1234/acknowledge",
-    "id": 1234,
+    "id": "f0d98b28-bf9d-49e7-b9d0-ac7cbb52e73a",
     "details": "Violation description test.",
     "alertPolicyNames": ["Test policy name"],
     "condition_name": "Server Down",

zerver/webhooks/newrelic/fixtures/incident_created_new.json

@@ -1,6 +1,6 @@
 {
     "incident_acknowledge_url": "https://alerts.newrelic.com/accounts/2941966/incidents/1234/acknowledge",
-    "id": 1234,
+    "id": "8114ada3-572e-4550-a310-12375371669e",
     "details": "Violation description test.",
     "alertPolicyNames": ["Test policy name"],
     "condition_name": "Server Down",

zerver/webhooks/newrelic/fixtures/incident_default_fields_new.json

@@ -1,5 +1,5 @@
 {
-    "id": 1234,
+    "id": "e04156e4-4cac-4f39-9d27-75d361e40a6d",
     "createdAt": 1605133931151,
     "state": "activated"
 }

zerver/webhooks/newrelic/fixtures/incident_malformatted_time_new.json

@@ -1,6 +1,6 @@
 {
     "incident_acknowledge_url": "https://alerts.newrelic.com/accounts/2941966/incidents/1234/acknowledge",
-    "id": 1234,
+    "id": "3881eb6c-6d57-447e-beaf-b2f610f014b0",
     "details": "Violation description test.",
     "alertPolicyNames": ["Test policy name"],
     "condition_name": "Server Down",

zerver/webhooks/newrelic/fixtures/incident_missing_state_new.json

@@ -1,6 +1,6 @@
 {
     "incident_acknowledge_url": "https://alerts.newrelic.com/accounts/2941966/incidents/1234/acknowledge",
-    "id": 1234,
+    "id": "279751d5-8ad8-41d4-adb8-9e895c58b606",
     "details": "Violation description test.",
     "alertPolicyNames": ["Test policy name"],
     "condition_name": "Server Down",

zerver/webhooks/newrelic/fixtures/incident_missing_timestamp_new.json

@@ -1,6 +1,6 @@
 {
     "incident_acknowledge_url": "https://alerts.newrelic.com/accounts/2941966/incidents/1234/acknowledge",
-    "id": 1234,
+    "id": "6eba9b54-898f-43eb-8892-e41ddca4a10f",
     "details": "Violation description test.",
     "alertPolicyNames": ["Test policy name"],
     "condition_name": "Server Down",

zerver/webhooks/newrelic/fixtures/incident_state_not_recognized_new.json

@@ -1,6 +1,6 @@
 {
     "incident_acknowledge_url": "https://alerts.newrelic.com/accounts/2941966/incidents/1234/acknowledge",
-    "id": 1234,
+    "id": "1d1fff5c-c723-4bf6-8458-bc63b253279e",
     "details": "Violation description test.",
     "alertPolicyNames": ["Test policy name"],
     "condition_name": "Server Down",

zerver/webhooks/newrelic/fixtures/incident_time_too_large_new.json

@@ -1,6 +1,6 @@
 {
     "incident_acknowledge_url": "https://alerts.newrelic.com/accounts/2941966/incidents/1234/acknowledge",
-    "id": 1234,
+    "id": "37d07862-e156-480f-86f9-7e8e9b7ed4f4",
     "details": "Violation description test.",
     "alertPolicyNames": ["Test policy name"],
     "condition_name": "Server Down",