Enable code host "sync all" mode for Sourcegraph cloud

Created by: artemruts

Description

Currently code host's "sync all" functionality is behind a AllowUserExternalServiceSyncAll user tag on the client. To enable it we need to investigate the following issue:

• https://github.com/sourcegraph/sourcegraph/issues/20478 — ExternalService endpoint doesn't return repos in allRepos:true mode

After the issues are resolved there might be a few straightforward code changes (https://github.com/sourcegraph/sourcegraph/issues/20570) on the client to make "sync all" work.