Add edited property to AppMentionEvent class

[seratch]

As pointed out at slackapi/bolt-js#960 , an app_mention event payload can have edited property as with message event payloads.

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

Merging #755 (1a7ad28) into main (7de2eb8) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main     #755   +/-   ##
=========================================
  Coverage     77.42%   77.42%           
  Complexity     3303     3303           
=========================================
  Files           370      370           
  Lines          9807     9807           
  Branches        935      935           
=========================================
  Hits           7593     7593           
  Misses         1662     1662           
  Partials        552      552           

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