build(deps): Bump gatsby-plugin-sitemap from 6.14.0 to 6.15.0

Bumps [gatsby-plugin-sitemap](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-sitemap) from 6.14.0 to 6.15.0.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-sitemap/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-plugin-sitemap@6.15.0/packages/gatsby-plugin-sitemap)

---
updated-dependencies:
- dependency-name: gatsby-plugin-sitemap
  dependency-version: 6.15.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

package-lock.json

@@ -27,7 +27,7 @@
     "gatsby-plugin-layout": "^4.14.0",
     "gatsby-plugin-matomo": "^0.17.0",
     "gatsby-plugin-sass": "^6.14.0",
-    "gatsby-plugin-sitemap": "^6.14.0",
+    "gatsby-plugin-sitemap": "^6.15.0",
     "gatsby-source-filesystem": "^5.14.0",
     "gatsby-source-graphql": "^5.14.0",
     "gatsby-transformer-csv": "^5.14.0",