build(deps): bump postcss and vue-loader in /build/frontend-legacy

Bumps [postcss](https://github.com/postcss/postcss) to 8.5.6 and updates ancestor dependency [vue-loader](https://github.com/vuejs/vue-loader). These dependencies need to be updated together.


Updates `postcss` from 7.0.39 to 8.5.6
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.39...8.5.6)

Updates `vue-loader` from 15.11.1 to 17.4.2
- [Release notes](https://github.com/vuejs/vue-loader/releases)
- [Changelog](https://github.com/vuejs/vue-loader/blob/main/CHANGELOG.md)
- [Commits](vuejs/vue-loader@v15.11.1...v17.4.2)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.6
  dependency-type: indirect
- dependency-name: vue-loader
  dependency-version: 17.4.2
  dependency-type: direct:development
...

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

Author: dependabot[bot]

build/frontend-legacy/package-lock.json

@@ -134,7 +134,7 @@
     "typescript": "^5.9.2",
     "vite": "^7.2.2",
     "vitest": "^3.2.4",
-    "vue-loader": "^15.11.1",
+    "vue-loader": "^17.4.2",
     "vue-template-compiler": "^2.7.16",
     "webpack": "^5.102.1",
     "webpack-cli": "^6.0.1",