Update dependency @mantine/core to v8

Author: renovate[bot]

example/package.json

@@ -15,7 +15,7 @@
     "@blocknote/mantine": "0.34.0",
     "@blocknote/react": "0.41.1",
     "@convex-dev/prosemirror-sync": "file:..",
-    "@mantine/core": "^7.5.3",
+    "@mantine/core": "^8.0.0",
     "@tiptap/core": "2.26.2",
     "@tiptap/extension-highlight": "2.26.2",
     "@tiptap/extension-image": "2.26.2",