docs: correct async getters and removable observer examples

[OskarEichler]

Description

One Line Summary

Use current asynchronous subscription getters, retain callback references for removal, fix the removeAliases syntax, and replace removed in-app-message handler APIs with addEventListener examples.

Compatibility and observable changes

Documentation only; no runtime behavior changes. The guide remains a v4-to-v5 migration reference rather than a promise of full latest-version API coverage.

Details

Motivation

The source audit reproduced the failure paths described below. This PR contains only the associated fix; unrelated audit changes are in separate PRs.

Scope

• MIGRATION_GUIDE.md

Testing

Six extracted migration snippets type-check against current SDK declarations; the baseline has five missing-reference, syntax, or obsolete-method errors.

Each code/tooling fix was also applied independently to upstream commit a70312207cf094ac361eaa9196c317acb175c2cd and passed its targeted external actual-source diagnostics. Documentation snippets were checked separately. Native diagnostic harnesses use bridge/SDK doubles and are not an end-to-end push test.

On the combined audit branch:

• Existing SDK suite: 5 files, 262 tests pass; unchanged 95% coverage thresholds pass.
• vp check: formatting, lint and type checks pass; native Spotless check passes.
• Both example apps: Android Debug and unsigned iOS Simulator builds pass.
• Both example apps: iOS and Android production Metro bundles pass.

No checked-in test files were added or modified; regression evidence comes from external diagnostic harnesses and the existing suite. No physical-device, live notification delivery, Appium/BrowserStack, or release-workflow execution is claimed. The no-location example's stale native lock was updated locally to resolve the current SDK for verification; generated locks are not part of this PR.

Checklist

• Required description sections completed.
• Scope and observable/API behavior explained.
• Diff reviewed and targeted regression checks run.
• Automated checks and device-testing limitations documented.