Migrate bs-callout to GlAlert in ...vue_shared/components/callout.vue

What does this MR do?

Migrate the bs-callout usage within the Feature Flags 'configure' modal. Migrate the bs-callout within Jobs alert (when required to display) to use GlAlert. Update The spec file for the feature_flag_modal to reflect the new component used.

This also removes the Callout.vue component (and associated spec file). This is possible as there are no further references in the code to that component.

Potential risk

Any areas still under development using the Callout component will have merge conflicts or generally fail to work. However, this seems very unlikely given the migration exercise.

Screenshots (strongly suggested)

Screen	Before	After
Feature flags 'Configure' modal
Jobs

@tauriedavis These are the screenshots where I have concerns around the spacing/margin below the alert

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required) Not applicable
• Code review guidelines
• Merge request performance guidelines Not applicable
• Style guides
• Database guides Not applicable
• Separation of EE specific content Not applicable

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 Not applicable

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~~ Not applicable
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods Not applicable
• Security reports checked/validated by a reviewer from the AppSec team Not applicable

Related to #242029 (closed)