Bump gatsby-plugin-postcss from 5.5.0 to 5.6.0 in /packages/website

Bumps [gatsby-plugin-postcss](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-postcss) from 5.5.0 to 5.6.0.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-postcss/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-plugin-postcss@5.6.0/packages/gatsby-plugin-postcss)

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

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

packages/website/package.json

@@ -20,7 +20,7 @@
     "gatsby-plugin-manifest": "4.5.2",
     "gatsby-plugin-matomo": "0.11.0",
     "gatsby-plugin-offline": "5.4.0",
-    "gatsby-plugin-postcss": "5.5.0",
+    "gatsby-plugin-postcss": "5.6.0",
     "gatsby-plugin-purgecss": "6.1.0",
     "gatsby-plugin-react-helmet": "5.5.0",
     "gatsby-plugin-robots-txt": "1.6.14",

packages/website/yarn.lock

@@ -6162,10 +6162,10 @@ gatsby-plugin-page-creator@^4.4.0:
     globby "^11.0.4"
     lodash "^4.17.21"
 
-gatsby-plugin-postcss@5.5.0:
-  version "5.5.0"
-  resolved "https://registry.yarnpkg.com/gatsby-plugin-postcss/-/gatsby-plugin-postcss-5.5.0.tgz#cffd84f402141b8366fd3b8788ed11cce1fea4ed"
-  integrity sha512-FFZMkDCY7QwOBeJDHXOBgAndcTYqssQTpuR/50VX8rRqE0asuMSb3ryVVhug0MXTngecKa5filef1ICFsCBWag==
+gatsby-plugin-postcss@5.6.0:
+  version "5.6.0"
+  resolved "https://registry.yarnpkg.com/gatsby-plugin-postcss/-/gatsby-plugin-postcss-5.6.0.tgz#ee696317eb7601b2e0f5c64b9f9d488376fdb65f"
+  integrity sha512-AjRLMWMFrmahNSmEYMDwFdDGkUfyDxxAJ2qtteJ3SRAO0Jgb0N9wJM7RkBhAs6H7YuovQIBxoUOo5xrMTlgM4A==
   dependencies:
     "@babel/runtime" "^7.15.4"
     postcss-loader "^4.3.0"