Make Alert::STATUSES constant private

What does this MR do?

Contributes #231349 (closed)

Encapsulate Alert's status implementation details. Provide methods to operate with the Alert's status by its name instead of raw values.

Good news are that we don't need to introduce breaking changes to GraphQL, because GraphQL queries and mutations already operate with the status names instead of raw values. Raw values were used only internally.

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Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
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• 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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• [-] 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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