Updates User API attribute is-site-admin to leverage is-admin#900

brandonc merged 3 commits intomainhashicorp/go-tfe:mainfrom

mfhodges/tf-10833-user-api-leverages-is-admin-attributehashicorp/go-tfe:mfhodges/tf-10833-user-api-leverages-is-admin-attributeCopy head branch name to clipboard