Take the image into Inkscape. Trace it there. Automatic tracing to black and white does respectable job. My example below is traced with treshold 0,82. The tracing dialog is available in the Path menu.
You probably want to edit some small holes and juts because the the original stroke in the drawing is now a part of the interior. That also can be fixed somehow by ofsetting the stroke in the tracing result.
Here's the tracing result as a screenshot. I removed the black fill and added a thin stroke to see better the outline. Only three stray black dots is removed. You can edit the tracing result after you release combined paths and groups. But as written, only three unwanted dots has been removed here.
Inkscape does not make your STL without some extension that I do not have nor know. You probably must save the trace as DXF or other common CAD format and goto a proper 3D program for the extrusion.
Before saving it is useful in Inkscape to turn off the automatic white background. Goto File > Document properties and turn on the checkerboard background.