C#: Add change-note.

Author: michaelnebel

csharp/ql/lib/change-notes/2025-03-21-string-interpolation.md

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+---
+category: minorAnalysis
+---
+* The *alignment* and *format* clauses in string interpolation expressions are now extracted. That is, in `$"Hello {name,align:format}"` *name*, *align* and *format* are extracted as children of the string interpolation *insert* `{name,align:format}`.