text: Title: WebClient's paginated response iterator does not work for admin.conversations.search API
  Body: Usually, `next_cursor` in an API response exists under `response_metadata` property. However, only admin.conversations.search API method's response has it at the top level: https://github.com/slackapi/java-slack-sdk/blob/main/slack-api-client/src/main/java/com/slack/api/methods/response/admin/conversations/AdminConversationsSearchResponse.java#L19
  
  Due to this inconsistency, this part in this Python SDK does not work for the API. https://github.com/slackapi/python-slack-sdk/blob/v3.7.0/slack_sdk/web/slack_response.py#L141-L145 We can improve the internals of the class to support the API too.
  
  * TODOs
    * [ ] Add unit tests to reproduce the pattern
    * [ ] Change the logic
  
  ### Category (place an `x` in each of the `[ ]`)
  
  - [x] **slack_sdk.web.WebClient (sync/async)** (Web API client)
  - [ ] **slack_sdk.webhook.WebhookClient (sync/async)** (Incoming Webhook, response_url sender)
  - [ ] **slack_sdk.models** (UI component builders)
  - [ ] **slack_sdk.oauth** (OAuth Flow Utilities)
  - [ ] **slack_sdk.socket_mode** (Socket Mode client)
  - [ ] **slack_sdk.audit_logs** (Audit Logs API client)
  - [ ] **slack_sdk.scim** (SCIM API client)
  - [ ] **slack_sdk.rtm** (RTM client)
  - [ ] **slack_sdk.signature** (Request Signature Verifier)
  
  ### Requirements
  
  Please read the [Contributing guidelines](https://github.com/slackapi/python-slack-sdk/blob/main/.github/contributing.md) and [Code of Conduct](https://slackhq.github.io/code-of-conduct) before creating this issue or pull request. By submitting, you are agreeing to those rules.
  