Bump @angular/platform-server in /packages/altair-app

Bumps [@angular/platform-server](https://github.com/angular/angular/tree/HEAD/packages/platform-server) from 20.3.19 to 20.3.21.
- [Release notes](https://github.com/angular/angular/releases)
- [Changelog](https://github.com/angular/angular/blob/main/CHANGELOG.md)
- [Commits](https://github.com/angular/angular/commits/v20.3.21/packages/platform-server)

---
updated-dependencies:
- dependency-name: "@angular/platform-server"
  dependency-version: 20.3.21
  dependency-type: direct:production
...

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

Author: dependabot[bot]

packages/altair-app/package.json

@@ -16,7 +16,7 @@
     "@angular/forms": "20.3.19",
     "@angular/platform-browser": "20.3.19",
     "@angular/platform-browser-dynamic": "20.3.19",
-    "@angular/platform-server": "20.3.19",
+    "@angular/platform-server": "20.3.21",
     "@angular/router": "20.3.19",
     "@apollo/client": "3.5.6",
     "@codemirror/autocomplete": "^6.20.0",