Enable autocomplete on Design reply form

What does this MR do?

Enables autocomplete for Design Reply form in Design View.

This is primarily to support emoji autocomplete (raised in #208973 (closed)), but also gives us MR, Issue etc. autocomplete for free.

Where's the tests?

Given that we're simply changing the value of this prop, and that this functionality should be tested in the MarkdownField component or similar (it's actually tested deep down => https://gitlab.com/gitlab-org/gitlab/blob/c9228f2d291452d478b086dd0c082c482c31a43c/spec/frontend/gfm_auto_complete_spec.js), tests should not be necessary for this change.

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