docs: Update last_event_id in migration guide

Update the migration guide to mention that `last_event_id` has been reintroduced.

Fixes GH-3118

Author: szokeasaurusrex

Diff for: MIGRATION_GUIDE.md

@@ -64,7 +64,8 @@ Looking to upgrade from Sentry SDK 1.x to 2.x? Here's a comprehensive list of wh
 - Removed support for Flask 0.\*.
 - Removed support for gRPC < 1.39.
 - Removed support for Tornado < 6.
-- Removed `last_event_id()` top level API. The last event ID is still returned by `capture_event()`, `capture_exception()` and `capture_message()` but the top level API `sentry_sdk.last_event_id()` has been removed.
+- ~~Removed `last_event_id()` top level API. The last event ID is still returned by `capture_event()`, `capture_exception()` and `capture_message()` but the top level API `sentry_sdk.last_event_id()` has been removed.~~
+  - Update: Version 2.2.0 reintroduced `last_event_id()`.
 - Removed support for sending events to the `/store` endpoint. Everything is now sent to the `/envelope` endpoint. If you're on SaaS you don't have to worry about this, but if you're running Sentry yourself you'll need version `20.6.0` or higher of self-hosted Sentry.
 - The deprecated `with_locals` configuration option was removed. Use `include_local_variables` instead. See https://docs.sentry.io/platforms/python/configuration/options/#include-local-variables.
 - The deprecated `request_bodies` configuration option was removed. Use `max_request_body_size`. See https://docs.sentry.io/platforms/python/configuration/options/#max-request-body-size.