Bump postcss from 8.2.10 to 8.2.12 in /build/javascript (#952)

Bumps [postcss](https://github.com/postcss/postcss) from 8.2.10 to 8.2.12.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/8.2.10...8.2.12)

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build/javascript/package.json

@@ -16,7 +16,7 @@
   },
   "devDependencies": {
     "cssnano": "^5.0.1",
-    "postcss": "^8.2.10",
+    "postcss": "^8.2.12",
     "postcss-cli": "^8.3.1"
   },
   "snowpack": {