fix(deps): bump gatsby-plugin-offline from 6.11.0 to 6.12.0

Bumps [gatsby-plugin-offline](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-offline) from 6.11.0 to 6.12.0.
- [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@6.12.0/packages/gatsby-plugin-offline)

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

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

package.json

@@ -17,7 +17,7 @@
     "gatsby-plugin-manifest": "^5.12.0",
     "gatsby-plugin-netlify": "^5.1.0",
     "gatsby-plugin-netlify-cms": "^7.12.0",
-    "gatsby-plugin-offline": "^6.11.0",
+    "gatsby-plugin-offline": "^6.12.0",
     "gatsby-plugin-postcss": "^6.12.0",
     "gatsby-plugin-react-helmet": "^6.12.0",
     "gatsby-plugin-sass": "^6.12.0",