Simplify Apple App Store integration

What does this MR do and why?

As suggested in gitlab-org/incubation-engineering/mobile-devops/feedback#10 (comment 1355460868), this MR adds the APP_STORE_CONNECT_API_KEY_IS_KEY_CONTENT_BASE64 environment variable to CI jobs when the Apple App Store integration is enabled. This environment variable simplifies the Fastlane configuration from:

app_store_connect_api_key(
  is_key_content_base64: true
)

to this:

app_store_connect_api_key

The integration will still work with the original configuration, so it is backward compatible.

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