Switch canvases.* APIs to application/json format for more stability

[seratch]

Summary

Our internal users detected an issue with the form data request for canvases API calls when the markdown data is relatively large. This pull request switches the underlying request body data format to JSON for even better compatibility with the server-side.

Testing

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 (Documents)
• /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

All modified and coverable lines are covered by tests ✅

Project coverage is 84.86%. Comparing base (5fb245a) to head (469a24e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1574      +/-   ##
==========================================
- Coverage   84.93%   84.86%   -0.08%     
==========================================
  Files         113      113              
  Lines       12599    12599              
==========================================
- Hits        10701    10692       -9     
- Misses       1898     1907       +9     

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