Bump gatsby-plugin-sass from 4.0.0 to 4.15.0

Bumps [gatsby-plugin-sass](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-sass) from 4.0.0 to 4.15.0.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-sass/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/[email protected]/packages/gatsby-plugin-sass)

---
updated-dependencies:
- dependency-name: gatsby-plugin-sass
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

package-lock.json

@@ -34,7 +34,7 @@
     "gatsby-plugin-react-helmet": "^4.0.8",
     "gatsby-plugin-remove-trailing-slashes": "^3.0.0",
     "gatsby-plugin-resolve-src": "^2.0.0",
-    "gatsby-plugin-sass": "4.0.0",
+    "gatsby-plugin-sass": "4.15.0",
     "gatsby-plugin-sitemap": "^4.11.0",
     "gatsby-source-filesystem": "^3.0.0",
     "gatsby-transformer-remark": "^4.12.0",