Bump gatsby-plugin-offline from 6.14.0 to 6.15.0 (#1197)

Bumps [gatsby-plugin-offline](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-offline) 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-offline/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-plugin-offline@6.15.0/packages/gatsby-plugin-offline)

---
updated-dependencies:
- dependency-name: gatsby-plugin-offline
  dependency-version: 6.15.0
  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

@@ -20,7 +20,7 @@
     "gatsby-plugin-image": "3.15.0",
     "gatsby-plugin-manifest": "5.15.0",
     "gatsby-plugin-mdx": "5.14.1",
-    "gatsby-plugin-offline": "6.14.0",
+    "gatsby-plugin-offline": "6.15.0",
     "gatsby-plugin-sharp": "5.15.0",
     "gatsby-plugin-typography": "5.14.0",
     "gatsby-remark-copy-linked-files": "6.15.0",