Bump gatsby-source-filesystem from 5.14.0 to 5.15.0 (#1212)

Bumps [gatsby-source-filesystem](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-source-filesystem) from 5.14.0 to 5.15.0.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-source-filesystem/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-source-filesystem@5.15.0/packages/gatsby-source-filesystem)

---
updated-dependencies:
- dependency-name: gatsby-source-filesystem
  dependency-version: 5.15.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

package-lock.json

@@ -28,7 +28,7 @@
     "gatsby-remark-prismjs": "7.15.0",
     "gatsby-remark-responsive-iframe": "6.15.0",
     "gatsby-remark-smartypants": "6.15.0",
-    "gatsby-source-filesystem": "5.14.0",
+    "gatsby-source-filesystem": "5.15.0",
     "gatsby-transformer-sharp": "5.15.0",
     "glamor": "2.20.40",
     "prismjs": "1.30.0",