Prettify JSON of sample alert payload

What does this MR do?

Prettifies saved sample alert payload so that it is easier for the user to edit it

To check navigate to Settings > Operations > Alerts expand section or the direct link will be localhost/group/project/-/settings/operations#js-alert-management-settings

Screenshots (strongly suggested)

Before	After

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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Related to #325824 (closed)