Update dependency get-stdin to v8 (postcss#327)

Co-authored-by: Renovate Bot <bot@renovateapp.com>

Author: renovate[bot]

package.json

@@ -22,7 +22,7 @@
     "chokidar": "^3.3.0",
     "dependency-graph": "^0.9.0",
     "fs-extra": "^9.0.0",
-    "get-stdin": "^7.0.0",
+    "get-stdin": "^8.0.0",
     "globby": "^11.0.0",
     "postcss": "^7.0.0",
     "postcss-load-config": "^2.0.0",