Add change note

Author: tamasvajk

csharp/ql/lib/change-notes/2023-12-07-stringbuilder.md

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+category: minorAnalysis
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+* The dataflow models for the `System.Text.StringBuilder` class have been reworked. New summaries have been added for `Append` and `AppendLine`. With the changes, we expect queries that use taint tracking to find more results when interpolated strings or `StringBuilder` instances are passed to `Append` or `AppendLine`.