Bump xml2js and gatsby-transformer-sharp

Removes [xml2js](https://github.com/Leonidas-from-XIV/node-xml2js). It's no longer used after updating ancestor dependency [gatsby-transformer-sharp](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-transformer-sharp). These dependencies need to be updated together.


Removes `xml2js`

Updates `gatsby-transformer-sharp` from 2.3.1 to 5.9.0
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-transformer-sharp/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-transformer-sharp@5.9.0/packages/gatsby-transformer-sharp)

---
updated-dependencies:
- dependency-name: xml2js
  dependency-type: indirect
- dependency-name: gatsby-transformer-sharp
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

package.json

@@ -12,7 +12,7 @@
     "gatsby-plugin-react-helmet": "^3.1.13",
     "gatsby-plugin-sharp": "^4.25.1",
     "gatsby-source-filesystem": "^2.1.35",
-    "gatsby-transformer-sharp": "^2.3.1",
+    "gatsby-transformer-sharp": "^5.9.0",
     "prop-types": "^15.7.2",
     "react": "^16.11.0",
     "react-dom": "^16.11.0",