Make the same change to chat.postMessage with #1274

[seratch]

This pull request applies the same change with #1274 to chat.postMessage API patterns. Actually, this is not a major problem for chat.postMessage at all because the API's rate limit is pretty generous. With that being said, having this change to it as well should improve the consistency of the SDK behavior.

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

• [ ] bolt (Bolt for Java)
• [ ] bolt-{sub modules} (Bolt for Java - optional modules)
• [x] slack-api-client (Slack API Clients)
• [ ] slack-api-model (Slack API Data Models)
• [ ] *slack-api--kotlin-extension** (Kotlin Extensions for Slack API Clients)
• [ ] slack-app-backend (The primitive layer of Bolt for Java)

Requirements

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (7c838c1) 74.49% compared to head (0a98fee) 74.53%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1275 +/- ## ============================================ + Coverage 74.49% 74.53% +0.04% - Complexity 4135 4137 +2 ============================================ Files 444 444 Lines 13108 13114 +6 Branches 1324 1325 +1 ============================================ + Hits 9765 9775 +10 + Misses 2570 2566 -4 Partials 773 773 ```

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