fix: packages/app/package.json to reduce vulnerabilities (#8100)

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-POSTCSS-5926692

Co-authored-by: snyk-bot <[email protected]>

Author: CompuIves

packages/app/package.json

@@ -138,7 +138,7 @@
     "gsap": "^1.20.3",
     "gulp": "^3.9.1",
     "gulp-filter": "^5.0.0",
-    "gulp-postcss": "^6.4.0",
+    "gulp-postcss": "^9.0.0",
     "gulp-rev": "^7.1.2",
     "hash-sum": "^1.0.2",
     "http-browserify": "^1.7.0",
@@ -176,7 +176,7 @@
     "overmind-react": "^28.0.0-1624124645626",
     "path-browserify": "1.0.1",
     "phoenix": "^1.5.3",
-    "postcss": "^7.0.26",
+    "postcss": "^8.4.31",
     "postcss-selector-parser": "^2.2.3",
     "posthtml": "^0.11.3",
     "posthtml-parser": "^0.4.1",