This rule was added in Rubocop 0.89.0. However, there are some false
positives when we don't use interpolation but simply concatenate in
order to avoid long lines. Even if there weren't false positives, there
are places where we concatenate to emphasize the point that we're adding
a certain character to a text.
We might reconsider this rule in the future, since we generally prefer
interpolation over concatenation.
