Add change note

Author: hvitved

csharp/ql/src/change-notes/2026-04-17-useless-to-string.md

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+---
+category: minorAnalysis
+---
+* The query `cs/useless-tostring-call` has been updated to avoid false
+  positive results in calls to `StringBuilder.AppendLine` and calls of
+  the form `base.ToString()`. Moreover, the alert message has been
+  made more precise.