ci(docs-infra): rename 'upload-server' to 'preview-server'

The server no longer has files uploaded to it. Instead it is more
accurate to refer to it as dealing with "previews" of PRs.

aio/aio-builds-setup/dockerbuild/scripts-js/lib/common/github-pull-requests.ts

@@ -1,5 +1,5 @@
-import {assert, assertNotMissingOrEmpty} from '../common/utils';
 import {GithubApi} from './github-api';
+import {assert, assertNotMissingOrEmpty} from './utils';
 
 export interface PullRequest {
   number: number;