Remove long-deprecated javax.servlet modules from slack-app-backend library

[seratch]

This pull request removes the "javax.servlet-api" artifact from the slack-app-backend library. The dependency is in "provided" scope and is only used by two long-deprecated classes. Those classes will be removed too:

• com.slack.api.app_backend.events.servlet.SlackEventsApiServlet
• com.slack.api.app_backend.events.servlet.SlackSignatureVerifier

The primary reason why we're considering this removal this time is that having the javax.servlet dependency can be a noise for developers and admins who are examining the progress of the Jakarta Servlet migration (even though it's in "provided" scope). Initially, I was planning to remove these deprecated classes in v2, but we don't have immediate plans to release v2. Therefore, we will delete these in a minor release and clearly mention it along with the migration path in the release notes.

For context, the early version of this project was my personal project called jSlack, and we migrated it to the java-slack-sdk back in 2020. These classes have been deprecated since this project was migrated to the SlackAPI organization.

Thus, I'm sure that even several years ago, almost 100% of users were already using bolt-java modules rather than these two. There might be very few applications relying on these deprecated classes. For those users, we recommend migrating to the newer and better bolt-java modules or copying the deprecated code into their projects, as these two are quite small.

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 73.26%. Comparing base (9b7774d) to head (0027585).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1412      +/-   ##
============================================
- Coverage     73.33%   73.26%   -0.07%     
+ Complexity     4387     4368      -19     
============================================
  Files           476      474       -2     
  Lines         14223    14172      -51     
  Branches       1434     1429       -5     
============================================
- Hits          10430    10383      -47     
- Misses         2949     2951       +2     
+ Partials        844      838       -6     

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