Remove deprecated xhr from specs

What does this MR do?

Fixes deprecation:

DEPRECATION WARNING: `xhr` and `xml_http_request` are deprecated and will be removed in Rails 5.1.
Switch to e.g. `post :create, params: { comment: { body: 'Honey bunny' } }, xhr: true`.

What are the relevant issue numbers?

Closes #55448 (closed).

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer