chore(dependencies): update dependency gh-pages to v6

Author: renovate[bot]

package-lock.json

@@ -20,7 +20,7 @@
   "devDependencies": {
     "@babel/preset-env": "7.12.13",
     "@stencila/dev-config": "2.0.27",
-    "gh-pages": "3.2.3"
+    "gh-pages": "6.3.0"
   },
   "renovate": {
     "extends": [