refactor(@angular/ssr): mark @angular/platform-server as optional

Whlist, this package is not really optional, NPM will install the wrong verson of peer dependencies when the Angular CLI is in prerelease mode.

```ts
STDERR:
npm error code ERESOLVE
npm error ERESOLVE unable to resolve dependency tree
npm error
npm error While resolving: [email protected]
npm error Found: @angular/[email protected]
npm error node_modules/@angular/animations
npm error   @angular/animations@"^19.1.0-next.0" from the root project
npm error
npm error Could not resolve dependency:
npm error peer @angular/animations@"19.0.1" from @angular/[email protected]
npm error node_modules/@angular/platform-server
npm error   peer @angular/platform-server@"^19.1.0-next.0 || ^19.0.0" from @angular/[email protected]
npm error   node_modules/@angular/ssr
npm error     @angular/ssr@"19.1.0-next.0" from the root project
npm error
npm error Fix the upstream dependency conflict, or retry
npm error this command with --force or --legacy-peer-deps
```

packages/angular/ssr/package.json

@@ -21,6 +21,11 @@
     "@angular/platform-server": "^19.0.0 || ^19.1.0-next.0",
     "@angular/router": "^19.0.0 || ^19.1.0-next.0"
   },
+  "peerDependenciesMeta": {
+    "@angular/platform-server": {
+      "optional": true
+    }
+  },
   "devDependencies": {
     "@angular/common": "^19.1.0-next.0",
     "@angular/compiler": "^19.1.0-next.0",