build(deps): bump gatsby-plugin-offline from 4.0.0 to 4.2.0

Bumps [gatsby-plugin-offline](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-offline) from 4.0.0 to 4.2.0.
- [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@4.2.0/packages/gatsby-plugin-offline)

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

package.json

@@ -9,7 +9,7 @@
     "gatsby-image": "3.0.0",
     "gatsby-plugin-htaccess": "1.4.0",
     "gatsby-plugin-manifest": "3.0.0",
-    "gatsby-plugin-offline": "4.0.0",
+    "gatsby-plugin-offline": "4.2.0",
     "gatsby-plugin-preact": "5.0.0",
     "gatsby-plugin-react-helmet": "4.0.0",
     "gatsby-plugin-sharp": "3.0.1",