Use path instead of URL for issue boards endpoint

The following discussion from !53100 (merged) should be addressed:

• @engwan started a discussion: (+1 comment)

  I see that we're using _url for group boards and epic boards. And _path for project boards.

  We should use _path here (and in the group boards too) because of https://docs.gitlab.com/ee/development/fe_guide/frontend_faq.html#5-should-i-use-a-full-url-ie-gongitlab_url-or-a-full-path-ie-gonrelative_url_root-when-referencing-backend-endpoints