use Image.LANCZOS instead of Image.ANTIALIAS for thumbnail resize (#15876)

Image.ANTIALIAS is not defined in current pillow releases. Since ANTIALIAS was just using LANCZOS anyways, this is just a cosmetic change, but makes synapse work with most recent pillow releases.

Signed-off-by: Giovanni Harting <539@idlegandalf.com>

changelog.d/15876.bugfix

@@ -0,0 +1 @@
+Correctly resize thumbnails with pillow version >=10.

synapse/media/thumbnailer.py

@@ -131,7 +131,7 @@ class Thumbnailer:
             else:
                 with self.image:
                     self.image = self.image.convert("RGB")
-        return self.image.resize((width, height), Image.ANTIALIAS)
+        return self.image.resize((width, height), Image.LANCZOS)
 
     def scale(self, width: int, height: int, output_type: str) -> BytesIO:
         """Rescales the image to the given dimensions.