add migration note

Author: Lms24

MIGRATION.md

@@ -314,6 +314,7 @@ For our efforts to reduce bundle size of the SDK we had to remove and refactor p
 - Remove `SDK_NAME` export from `@sentry/browser`, `@sentry/node`, `@sentry/tracing` and `@sentry/vue` packages.
 - Removed `eventStatusFromHttpCode` to save on bundle size.
 - Replace `BrowserTracing` `maxTransactionDuration` option with `finalTimeout` option
+- Replace `Session` class with a session object and functions (see [#5054](https://github.com/getsentry/sentry-javascript/pull/5054)).
 
 ## Sentry Angular SDK Changes