[Fix #12703] Fix an error for Lint/MixedCaseRange with invalid byte sequence in UTF-8

[Earlopain]

Closes #12703

While this fixes the above issues, there is still a problem with this cop. If a regex contains this escape and also contains an offense, it will still error. I have openend #12750 for this. Fixing this as well turned out to be too complicated for me.

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• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
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[koic]

Thanks!