Unclear messaging on Ruby version check

[elliotcm]

When running an older version of rubocop against a newly released Ruby version, this error is output:

Error: Unknown Ruby version 2.6 found in .ruby-version.

That's fine, but the issue is that when it appears in the normal flow of a linting run it's not clear that rubocop is what's complaining, and it often looks more like e.g. rbenv complaining that the Ruby version isn't installed on the system.

Would it be possible to rephrase this in such a way that it's clear that it's rubocop (or a linter in general) that doesn't know about the stated Ruby version so the remedial steps (upgrading rubocop) are clearer to the reader?

I didn't want to open a PR on this one since I didn't know how best you'd want to indicate this, but I'm happy to open one after some feedback on wording.

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