Upgrade fabric to 6.0.0-beta20 (#30121)

Fixes https://github.com/go-gitea/gitea/issues/29326 because it includes
https://github.com/fabricjs/fabric.js/pull/9707.

Makefile

 
 .PHONY: generate-images
 generate-images: | node_modules
-	npm install --no-save fabric@6.0.0-beta19 imagemin-zopfli@7
+	npm install --no-save fabric@6.0.0-beta20 imagemin-zopfli@7
 	node tools/generate-images.js $(TAGS)
 
 .PHONY: generate-manpage