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)
• 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@             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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