Allow compatibility with Kotlin 1.6+

[rocketraman]

Resolves #1277 .

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)
• [X] *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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[rocketraman]

@seratch The JDK 14 test failure doesn't seem related to this. Any thoughts?

[seratch]

Will re-run it

[codecov[bot]]

Codecov Report

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

Comparison is base (4c95cd4) 74.24% compared to head (ad58a9a) 74.30%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1278 +/- ## ============================================ + Coverage 74.24% 74.30% +0.06% - Complexity 4124 4130 +6 ============================================ Files 444 444 Lines 13114 13114 Branches 1325 1325 ============================================ + Hits 9737 9745 +8 + Misses 2604 2600 -4 + Partials 773 769 -4 ```

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[seratch]

It’s successful now! Thank you so much again for your time!