Replace v-html with v-safe-html in delete_user_modal.vue

What does this MR do?

This MR replaces v-html with v-safe-html in app/assets/javascripts/pages/admin/users/components/delete_user_modal.vue.

Relates to #241921 (closed).

Screenshots

The affected component is the user delete modal in the admin panel.

1. Go to the admin dashboard
2. Go to a user page
3. Click on "Delete user and contributions"
4. Check that the modal is rendered correctly (especially the text above the input).

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