Recommend using the files_upload_v2 method for uploading files

[zimeg]

Summary

This PR updates the example in the README to recommend using files_upload_v2 for uploading files to avoid warnings raised by the current files_upload method.

Notes

Unsure if linking to the release notes for files_upload_v2 is alright (instead of API documentation) since there are multiple API methods being called now. These release notes do a great job of documenting this method though, so I think this might be helpful!

Category

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.signature (Request Signature Verifier)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.models (UI component builders)
• slack_sdk.scim (SCIM API client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.rtm_v2 (RTM client)
• /docs-src (Documents, have you run ./scripts/docs.sh?)
• /docs-src-v2 (Documents, have you run ./scripts/docs-v2.sh?)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run python3 -m venv .venv && source .venv/bin/activate && ./scripts/run_validation.sh after making the changes.

[codecov]

Codecov Report

Merging #1402 (997eb3b) into main (91c22ea) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1402      +/-   ##
==========================================
+ Coverage   85.37%   85.40%   +0.03%     
==========================================
  Files         111      111              
  Lines       11860    11860              
==========================================
+ Hits        10125    10129       +4     
+ Misses       1735     1731       -4     

see 1 file with indirect coverage changes

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[seratch]

Thanks! LGTM