Upgrade fabric to 6.0.0-beta20 (#30121)

Fixes #29326 because it includes
fabricjs/fabric.js#9707.

Author: silverwind

Makefile

@@ -959,7 +959,7 @@ generate-gitignore:
 
 .PHONY: generate-images
 generate-images: | node_modules
-	npm install --no-save [email protected]beta19 imagemin-zopfli@7
+	npm install --no-save [email protected]beta20 imagemin-zopfli@7
 	node tools/generate-images.js $(TAGS)
 
 .PHONY: generate-manpage