Update vae config (use_legacy -> use_quant_conv)

See https://github.com/huggingface/diffusers/pull/7005#discussion_r1508530077

vae/config.json

@@ -20,5 +20,5 @@
   "out_channels": 3,
   "sample_size": 768,
   "scaling_factor": 0.18215,
-  "use_legacy": false
+  "use_quant_conv": false
 }