Update dependency gatsby-transformer-remark to v5 [SECURITY]

Author: renovate[bot]

package.json

@@ -34,7 +34,7 @@
     "gatsby-remark-images": "5.5.0",
     "gatsby-source-contentful": "5.8.0",
     "gatsby-source-filesystem": "3.8.0",
-    "gatsby-transformer-remark": "4.5.0",
+    "gatsby-transformer-remark": "5.25.1",
     "gatsby-transformer-sharp": "3.8.0",
     "gensync": "1.0.0-beta.2",
     "immutability-helper": "3.1.1",