Disable sign up in alerta (monitoring) (#1129)

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## What this PR does


### Release note

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```release-note
[]
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## Summary by CodeRabbit

* **New Features**
* Added a new environment variable to the monitoring alert system to
control signup availability.

* **Chores**
  * Updated the monitoring package version to 1.12.0.
* Revised version mapping for improved tracking of monitoring package
releases.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/extra/monitoring/Chart.yaml

@@ -3,4 +3,4 @@ name: monitoring
 description: Monitoring and observability stack
 icon: /logos/monitoring.svg
 type: application
-version: 1.11.0
+version: 1.12.0

packages/extra/monitoring/templates/alerta/alerta.yaml

@@ -104,7 +104,8 @@ spec:
                 secretKeyRef:
                   name: alerta-db-app
                   key: uri
-
+            - name: SIGNUP_ENABLED
+              value: "False"
             - name: AUTH_REQUIRED
               value: "True"
 

packages/extra/versions_map

@@ -45,7 +45,8 @@ monitoring 1.9.1 fd240701
 monitoring 1.9.2 f9f8bb2f
 monitoring 1.10.0 632224a3
 monitoring 1.10.1 8c86905b
-monitoring 1.11.0 HEAD
+monitoring 1.11.0 4369b031
+monitoring 1.12.0 HEAD
 seaweedfs 0.1.0 71514249
 seaweedfs 0.2.0 5fb9cfe3
 seaweedfs 0.2.1 fde4bcfa