docs: update ts-loader version in migration guide [skip ci]

Author: haoqunjiang

docs/migrations/migrate-from-v4.md

@@ -142,7 +142,7 @@ The [`transpileDependencies` option](../config/#transpiledependencies) now accep
 
 * Dropped TSLint support. As [TSLint has been deprecated](https://github.com/palantir/tslint/issues/4534), we [removed](https://github.com/vuejs/vue-cli/pull/5065) all TSLint-related code in this version.
 Please consider switching to ESLint. You can check out [`tslint-to-eslint-config`](https://github.com/typescript-eslint/tslint-to-eslint-config) for a mostly automatic migration experience.
-* `ts-loader` is upgraded from v6 to v8. It now only supports TypeScript >= 3.6.
+* `ts-loader` is upgraded from v6 to v9. It now only supports TypeScript >= 3.6.
 * `fork-ts-checker-webpack-plugin` is upgraded from v3.x to v6.x, you can see the detailed breaking changes in its release notes:
   * [v4.0.0](https://github.com/TypeStrong/fork-ts-checker-webpack-plugin/releases/tag/v4.0.0)
   * [v5.0.0](https://github.com/TypeStrong/fork-ts-checker-webpack-plugin/releases/tag/v5.0.0)