AvoidLongLines: Make internal function DiagnosticSeverity private

[bergmeister]

PR Summary

Code analysis showed that this function is only used by the rule itself, therefore it shouldn't have been public. No change of behavior. Technically breaking but this method was never intended to be public and other rules do not have public methods to return the DiagnosticSeverity they will emit, in fact depending on the analysis they might be different.

PR Checklist

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• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
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