Fixed unresponsive downloads.sf.net urls

I had trouble installing `imagemagick` as the download for `libpng`, `freetype` and `imagemagick` itself were timeouting.

I used `downloads.sourceforge.net` instead of `downloads.sf.net` and managed to successfully download them.
According to https://github.com/Homebrew/homebrew/pull/20645 it's the recommended SourceForge host to use.

I updated a few other formulas that were referencing `downloads.sf.net`.