chore: remove deprecated appcenter references

[Andries-Smit]

Description

Removes all deprecated AppCenter (Microsoft's mobile CI/CD service) references from the repository. AppCenter is no longer used for builds or OTA updates. This cleans up dead files and configuration that have been superseded.

Deleted files:

• appcenter-build-config.json — AppCenter build configuration for iOS and Android
• appcenter-pre-build.sh — pre-build hook script
• appcenter-post-clone.sh — post-clone hook script (root)
• ios/appcenter-post-clone.sh — post-clone hook script (iOS)

Modified files:

• capabilities.ios.json / capabilities.android.json — removed appCenterOTA property (was already false)
• .github/workflows/ios-build-custom-dev-app.yml — removed the step that tested appcenter-pre-build.sh

Checklist

• Release Note: As a part of the release process, an automated PR will be created for the Mendix Docs repository (https://github.com/mendix/docs) relevant to the changes introduced in this PR. Post release, please ensure that the release note accurately reflects the changes and impacts of this PR.
• Breaking Changes: This PR introduces breaking changes (e.g., changes that require updates to existing configurations, dependencies).
  • If yes, I have documented these breaking changes and provided guidance for users to adapt.
  • Details about breaking changes: N/A — AppCenter is no longer in use; no active builds relied on these files.

This PR contains

• Bug fix
• Feature
• Refactor
• Other (describe)

Cleanup of deprecated AppCenter CI/CD integration files and configuration.

Important Notes

• Make sure the release note accurately describes the changes and impacts introduced by this PR.

What should be covered while testing?

• Verify the iOS CI workflow (ios-build-custom-dev-app.yml) completes successfully without the removed AppCenter step.
• Confirm capabilities.ios.json and capabilities.android.json are valid JSON and CocoaPods/Gradle build steps remain unaffected.

Extra comments (optional)

Historical references to AppCenter in CHANGELOG.md and .mx/releases_list.json were intentionally left in place as they document past release history.