Move User.alert_bot out of EE

What does this MR do?

Contributes to #42640 (closed)

Implements #200006 (closed)

User.alert_bot is one of the dependencies for moving Alerting functionality to the Core. More information about all the dependencies is described here.

User.alert_bot is a dependency, but "support" and "visual review" bots aren't. Although, because all bots share the same enum that makes them harder to separate. Since "support" and "visual review" bots are used only in EE codebase, I think that's not a problem to define them in CE.

Screenshots

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

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