update migration docs with api changes

Author: AbhiPrasad

MIGRATION.md

@@ -56,27 +56,26 @@ However, directly importing from specific files is discouraged.
 
 ## Removing the `API` class from `@sentry/core`
 
-The internal `API` class was removed in favor of the `initAPIDetails` function and the `APIDetails` type. More details can be found in the [PR that deprecated this class](https://github.com/getsentry/sentry-javascript/pull/4281). To migrate, see the following example.
+The internal `API` class was removed in favor of explicitly accessing client options and the dsn.
 
 ```js
 // New in v7:
-import {
-  initAPIDetails,
-  getEnvelopeEndpointWithUrlEncodedAuth,
-  getStoreEndpointWithUrlEncodedAuth,
-} from '@sentry/core';
+import { makeDsn } from '@sentry/utils';
 
-const dsn = initAPIDetails(dsn, metadata, tunnel);
+const dsn = makeDsn(dsn);
 const dsn = api.dsn;
-const storeEndpoint = getEnvelopeEndpointWithUrlEncodedAuth(api.dsn, api.tunnel);
-const envelopeEndpoint = getStoreEndpointWithUrlEncodedAuth(api.dsn);
+
+const client = getCurrentHub().getClient();
+const dsn = client.getDsn();
+const clientOptions = client.getOptions();
+
+const envelopeEndpoint = getEnvelopeEndpointWithUrlEncodedAuth(dsn, clientOptions.tunnel);
 
 // Before:
 import { API } from '@sentry/core';
 
 const api = new API(dsn, metadata, tunnel);
 const dsn = api.getDsn();
-const storeEndpoint = api.getStoreEndpointWithUrlEncodedAuth();
 const envelopeEndpoint = api.getEnvelopeEndpointWithUrlEncodedAuth();
 ```