Polish error message when 2fa is required

The following discussion from !84151 (merged) should be addressed in a post-merge review:

• @dblessing started a discussion: (+3 comments)

  Question: Should we mention that a Personal Access Token must be used when 2FA is enabled?

File in question: app/controllers/concerns/enforces_two_factor_authentication.rb

Line to review: render_error("2FA required", status: :unauthorized)