DOC: behavior change note streamplot no varying width/color

Author: gokberkgunes

doc/api/next_api_changes/behavior/25112-GG.rst

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+Streamplot now draws streamlines as one piece if no width or no color variance
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Since there is no need to draw streamlines piece by piece if there is no color
+change or width change, now streamplot will draw each streamline in one piece.
+
+The behavior for varying width or varying color is not changed, same logic is
+used for these kinds of streamplots.