docs(plugin): bump minimum gatsby version to 3.0

BREAKING CHANGE: The plugin now requires at least Gatsby v3.0.4

README.md

@@ -278,6 +278,6 @@ These events are emitted at certain run time points.
 [codecov-url]: https://codecov.io/github/mohatt/gatsby-plugin-postbuild
 [codecov-img]: https://img.shields.io/codecov/c/github/mohatt/gatsby-plugin-postbuild.svg?logo=codecov&logoColor=white
 [gatsby-url]: https://www.gatsbyjs.org/packages/gatsby-plugin-postbuild
-[gatsby-img]: https://img.shields.io/badge/gatsby->=2.25-blueviolet.svg?logo=gatsby
+[gatsby-img]: https://img.shields.io/badge/gatsby->=3.0-blueviolet.svg?logo=gatsby
 [license-url]: https://github.com/mohatt/gatsby-plugin-postbuild/blob/master/LICENSE
 [license-img]: https://img.shields.io/github/license/mohatt/gatsby-plugin-postbuild.svg?logo=open%20source%20initiative&logoColor=white

package.json

@@ -81,7 +81,7 @@
     "typescript": "4.2.4"
   },
   "peerDependencies": {
-    "gatsby": ">=2.25.4",
+    "gatsby": ">=3.0.4",
     "postcss": ">=8.2.1"
   },
   "engines": {