Use stable version of fabric (#31526)

Fabric 6 is [now
stable](https://github.com/fabricjs/fabric.js/releases/tag/v6.0.0-rc5),
use it.

Makefile

@@ -969,7 +969,7 @@ generate-gitignore:
 
 .PHONY: generate-images
 generate-images: | node_modules
-	npm install --no-save fabric@6.0.0-beta20 imagemin-zopfli@7
+	npm install --no-save fabric@6 imagemin-zopfli@7
 	node tools/generate-images.js $(TAGS)
 
 .PHONY: generate-manpage