Fix webpack 4 support by setting `target: es2017` for `redux.legacy-esm.js`

[aryaemami59]

Overview

Even though this change does not make a difference in output, it would still be good to correct this now just so it doesn't become an issue later on.

This PR:

• [X] Fixes webpack 4 support by setting target: es2017 for redux.legacy-esm.js which is related to https://github.com/reduxjs/redux-toolkit/issues/4282 and https://github.com/reduxjs/redux/pull/4687.

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[timdorr]

Thanks!