feat: add admin.users.getExpiration method

[srtaalej]

Summary

Adds the missing admin.users.getExpiration Web API method to WebClient (sync, async, and legacy via codegen). It takes a required user_id and an optional target_team, and is called with GET as documented.

Fixes #1982

Testing

• ./scripts/run_tests.sh tests/slack_sdk_async/web/test_web_client_coverage.py passes with the new method registered in the coverage test
• ./scripts/lint.sh passes

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 84.30%. Comparing base (5029dac) to head (ac0887b).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1983      +/-   ##
==========================================
+ Coverage   84.28%   84.30%   +0.01%     
==========================================
  Files         118      118              
  Lines       13679    13688       +9     
==========================================
+ Hits        11530    11539       +9     
  Misses       2149     2149              

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