fix(deps): update dependency postcss-loader to v7 for gatsby-plugin-postcss (#33506)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

packages/gatsby-plugin-postcss/package.json

@@ -8,7 +8,7 @@
   },
   "dependencies": {
     "@babel/runtime": "^7.15.4",
-    "postcss-loader": "^4.3.0"
+    "postcss-loader": "^7.0.1"
   },
   "devDependencies": {
     "@babel/cli": "^7.15.4",