Add section on TSerialized generic removal (#12949)

Author: jerelmiller

.size-limits.json

@@ -1,6 +1,6 @@
 {
-  "import { ApolloClient, InMemoryCache, HttpLink } from \"@apollo/client\" (CJS)": 43931,
-  "import { ApolloClient, InMemoryCache, HttpLink } from \"@apollo/client\" (production) (CJS)": 38692,
-  "import { ApolloClient, InMemoryCache, HttpLink } from \"@apollo/client\"": 33440,
-  "import { ApolloClient, InMemoryCache, HttpLink } from \"@apollo/client\" (production)": 27529
+  "import { ApolloClient, InMemoryCache, HttpLink } from \"@apollo/client\" (CJS)": 43936,
+  "import { ApolloClient, InMemoryCache, HttpLink } from \"@apollo/client\" (production) (CJS)": 38686,
+  "import { ApolloClient, InMemoryCache, HttpLink } from \"@apollo/client\"": 33476,
+  "import { ApolloClient, InMemoryCache, HttpLink } from \"@apollo/client\" (production)": 27519
 }

docs/source/migrating/apollo-client-4-migration.mdx

@@ -1567,6 +1567,17 @@ new ApolloClient({ // [!code ++]
 });
 ```
 
+### Removal of `<TSerialized>` generic argument
+
+The `TSerialized` generic argument has been removed from the `ApolloCache` abstract class. Most users set this type to `any` which provided little benefit for type safety. APIs that previously relied on `TSerialized` are now set to `unknown`.
+
+To migrate, remove the `TSerialized` generic argument when referencing `ApolloCache`.
+
+```ts
+const cache: ApolloCache<any> = ... // [!code --]
+const cache: ApolloCache = ... // [!code ++]
+```
+
 ## Apollo Link changes
 
 ### New class-based links