Docs: Update language around AI apps

[haleychaas]

We're phasing out the use of "AI apps". Using this phrasing creates an artificial boundary that leads to unnecessary complexity and confusion about support of AI features in Slack apps. I used "Agents & AI Apps" in places because that's what the feature is called in app settings. This is a docs-only change.

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 ✅

Project coverage is 72.92%. Comparing base (7475550) to head (fca9914).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1489      +/-   ##
============================================
+ Coverage     72.87%   72.92%   +0.04%     
- Complexity     4372     4374       +2     
============================================
  Files           475      475              
  Lines         14244    14244              
  Branches       1447     1447              
============================================
+ Hits          10380    10387       +7     
+ Misses         3008     3000       -8     
- Partials        856      857       +1     

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