[v2] Fix #422 invalid fields in MultiUsersSelectAction

[seratch]

Summary

This pull request brings #423 to the v2 branch.

Requirements (place an x in each [ ])

• 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.

[codecov]

Codecov Report

Merging #424 into @slack/bolt@next will not change coverage by %.
The diff coverage is n/a.

@@                Coverage Diff                @@
##           @slack/bolt@next     #424   +/-   ##
=================================================
  Coverage             85.51%   85.51%           
=================================================
  Files                     7        7           
  Lines                   511      511           
  Branches                146      146           
=================================================
  Hits                    437      437           
  Misses                   49       49           
  Partials                 25       25           

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