fix: update dependency github-slugger to ^1.2.1 (#16879)

Author: renovate[bot]

packages/gatsby-remark-autolink-headers/package.json

@@ -8,7 +8,7 @@
   },
   "dependencies": {
     "@babel/runtime": "^7.0.0",
-    "github-slugger": "^1.1.1",
+    "github-slugger": "^1.2.1",
     "lodash": "^4.17.14",
     "mdast-util-to-string": "^1.0.2",
     "unist-util-visit": "^1.3.0"