Add changelog

doc/modules/changes/20250304_gassmoeller

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+Fixed: The statistics file contained an invalid number of iterations
+for variables that were not solved (e.g. because compositions were
+prescribed, or Stokes was only solved using cheap or only expensive
+iterations). This is confusing, because it looks like an error and for the
+Stokes solver it caused incorrect iteration number to be reported. It is more
+accurate to report 0 iterations if none were made, which is what the
+postprocessor does now.
+<br>
+(Rene Gassmoeller, 2025/03/04)