build(docs-infra): add missing redirect rule for api/http in `fireb…

…ase.json` (#42452)

Previously, only sub-paths, such as `api/http/foo`, were redirected to
`guide/deprecations#http`.

This commit ensures that also `api/http` itself (which used to point to
the `http` module's API page) is redirected as well.

PR Close #42452

aio/firebase.json

@@ -67,6 +67,7 @@
       {"type": 301, "source": "/**/api/common/NgModel", "destination": "/api/forms/NgModel"},
 
       // `@angular/http` package was removed, and new `HttpClient` APIs are available under `@angular/common/http` package
+      {"type": 301, "source": "/api/http", "destination": "/guide/deprecations#http"},
       {"type": 301, "source": "/api/http/**", "destination": "/guide/deprecations#http"},
 
       // Animations moves, renames and removals

aio/tests/deployment/shared/URLS_TO_REDIRECT.txt

@@ -24,6 +24,7 @@
 /api/core/testing/inject-function.html --> /api/core/testing/inject
 /api/foo/bar/HTTP_PROVIDERS --> /guide/deprecations#http
 /api/foo/bar/HTTP_PROVIDERS_AND_MORE --> /guide/deprecations#http
+/api/http --> /guide/deprecations#http
 /api/http/Headers-class --> /guide/deprecations#http
 /api/http/Headers-class.html --> /guide/deprecations#http
 /api/http/HTTP_PROVIDERS-let --> /guide/deprecations#http