Update files.upload v2 method in correspondence with server-side changes

[seratch]

Summary

This pull request updates the internals of files_upload_v2 method to eliminate files.info API calls, which are no longer necessary because the server-side now returns the same metadata as part of files.completeUploadExternal API responses.

Category (place an x in each of the [ ])

• 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 (place an x in each [ ])

• 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 #1408 (f63e25e) into main (50c9265) will increase coverage by 0.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1408      +/-   ##
==========================================
+ Coverage   85.41%   85.46%   +0.05%     
==========================================
  Files         111      111              
  Lines       11871    11857      -14     
==========================================
- Hits        10140    10134       -6     
+ Misses       1731     1723       -8     

Files	Coverage Δ
slack_sdk/web/internal_utils.py	82.72% <ø> (+4.67%)	⬆️
slack_sdk/web/async_client.py	81.94% <0.00%> (ø)
slack_sdk/web/client.py	83.13% <0.00%> (ø)
slack_sdk/web/legacy_client.py	82.88% <0.00%> (ø)

... and 1 file with indirect coverage changes

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