Extract Merge Request Conflicts warning to component

What does this MR do?

For #232820 (closed)

There are a lot of potential asks in the final design solution. Some of them are stretch goals, but to make this work more consistent and avoid tech debt, the changes in this MR are a good, small, first step.

This MR extracts a warning that's shown on the MR Diffs app (Changes tab) so that it is a sub-component of the parent application. This matches the other warning shown in this screen (a component), and sets a precedent for future warnings (the final solution for #232820 (closed) requires a third warning).

Screenshots

There are no visual changes since the behavior and UI are the same.

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

Please note that because this component doesn't contain any logic, it also doesn't have any special tests.

Any tests for this new component would be testing either:

• that Vue itself works correctly, or
• that JavaScript (assignments, booleans) work correctly

• 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