Update dependency postcss-import to v15 (#439)

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

Author: renovate[bot]

package.json

@@ -38,7 +38,7 @@
     "eslint": "^8.2.0",
     "eslint-config-problems": "6.0.0",
     "postcss": "^8.0.4",
-    "postcss-import": "^14.0.0",
+    "postcss-import": "^15.0.0",
     "prettier": "~2.7.0",
     "sugarss": "^4.0.0",
     "uuid": "^9.0.0"