fix: update release instructions in maintainers guide

[WilliamBergamin]

Summary

Fix release instructions in maintainers_guide.md

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

All modified and coverable lines are covered by tests ✅

Project coverage is 85.06%. Comparing base (41cbaa4) to head (55535bb).

❗ Current head 55535bb differs from pull request most recent head 1c7e41a

Please upload reports for the commit 1c7e41a to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1507      +/-   ##
==========================================
- Coverage   85.08%   85.06%   -0.03%     
==========================================
  Files         112      112              
  Lines       12320    12320              
==========================================
- Hits        10483    10480       -3     
- Misses       1837     1840       +3     

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[seratch]

This change looks good to me. Could you add one more addition? Closing the GitHub milestone for the release is missing in the document. When making a new release, its GH milestone needs to be closed as well (I did for you this time: https://github.com/slackapi/python-slack-sdk/milestones?state=closed)

[seratch]

Also, creating the next patch version is preferred. I mean I've closed 3.28.0 and created 3.28.1 for forthcoming issues and PRs.