chore: Update migration docs to indicate sentry-migr8 is experimental. (#8988)

There was some confusion about this because the migration doc in this
repo conflicts with the `highly experimental` tag we put on the README.
Also added Node 18+ requirements.

We can change this in migr8 for a future release, but for now let's
merge this in to reduce confusion and unblock our users.

Author: AbhiPrasad

MIGRATION.md

@@ -1,6 +1,6 @@
 # Deprecations in 7.x
 
-You can use [@sentry/migr8](https://www.npmjs.com/package/@sentry/migr8) to automatically update your SDK usage and fix most deprecations:
+You can use the **Experimental** [@sentry/migr8](https://www.npmjs.com/package/@sentry/migr8) to automatically update your SDK usage and fix most deprecations. This requires Node 18+.
 
 ```bash
 npx @sentry/migr8@latest