refactor: change scalingMethod parameter to automatic for thumbnail generation (#6563)

#### What type of PR is this?

/area core
/kind improvement
/milestone 2.19.0

#### What this PR does / why we need it:

将缩略图生成的模式改为自动，速度优先会导致图片失真率较高，部分图片在 w1600 的尺寸下看起来会比较模糊。

#### Does this PR introduce a user-facing change?

```release-note
None
```

application/src/main/java/run/halo/app/core/attachment/ThumbnailGenerator.java

@@ -74,7 +74,7 @@ public class ThumbnailGenerator {
             throw new UnsupportedOperationException(
                 "Unsupported image format for: " + formatNameOpt.orElse("unknown"));
         }
-        var thumbnail = Scalr.resize(img, Scalr.Method.SPEED, Scalr.Mode.FIT_TO_WIDTH,
+        var thumbnail = Scalr.resize(img, Scalr.Method.AUTOMATIC, Scalr.Mode.FIT_TO_WIDTH,
             size.getWidth());
         var formatName = formatNameOpt.orElse("jpg");
         var thumbnailFile = getThumbnailFile(formatName);