Aspire 13.3 unified previously divergent polyglot withEnvironment APIs into a single withEnvironment(name, value) shape while retaining compatibility shims for older call sites. This PR updates AppHost docs and 13.3 release notes to reflect the unified signature, shim mappings, and migration direction.

• AppHost docs: unified API + migration guidance

  • Updated app-host/executable-resources.mdx with a dedicated Aspire 13.3 section covering:
    • unified withEnvironment(name, value) usage
    • supported value kinds
    • compatibility shim mappings
    • incremental migration guidance for existing code

• 13.3 changelog: explicit unification entry + before/after

  • Expanded whats-new/aspire-13-3.mdx with:
    • a top-level App model/AppHost note for WithEnvironment unification and shims
    • a concrete before/after comparison
    • explicit shim-to-unified mapping

// Before (older helpers)
await api.withEnvironmentEndpoint('SERVICE_URL', cache.primaryEndpoint);
await api.withEnvironmentParameter('API_KEY', apiKeyParam);
await api.withEnvironmentConnectionString('DB', database);

// After (unified API)
await api.withEnvironment('SERVICE_URL', cache.primaryEndpoint);
await api.withEnvironment('API_KEY', apiKeyParam);
await api.withEnvironment('DB', database);