Draft: Let AlertPresenter return specific presenter for generic or prometheus

What does this MR do?

This is a follow-up triggered by !38881 (comment 395429351).

This MR

• renames current AlertManagement::AlertPresenter to AlertManagement::GenericAlertPresenter (aligning with AlertManagement::PrometheusAlertPresenter)
• creates AlertManagement::AlertPresenter which returns the specific presenter (GenericAlertPresenter or PrometheusAlertPresenter)
• removes superfluous AlertManagement::Alert#present and the delegate

We'll wait until !38881 (merged) is merged hence Draft:.

Manual testing

Additionally to the spec I've tested both Alert integrations (Generic alerts and Prometheus alerts) locally and both still work fine.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] 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
• [-] Security reports checked/validated by a reviewer from the AppSec team