Bump postcss from 8.2.5 to 8.2.6 in /build/javascript (#709)

Bumps [postcss](https://github.com/postcss/postcss) from 8.2.5 to 8.2.6.
- [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/8.2.5...8.2.6)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

build/javascript/package.json

@@ -16,7 +16,7 @@
   },
   "devDependencies": {
     "cssnano": "^4.1.10",
-    "postcss": "^8.2.5",
+    "postcss": "^8.2.6",
     "postcss-cli": "^8.3.1"
   },
   "snowpack": {