Load mapping file in google play console

1. Click on "Releases Overview" in the left panel.
2. In the "Latest Releases" section, click on the version for which you want to upload your mapping file
3. In the "App Bundles and APKs" section, you would see a row with your added App Bundle or APK.
4. Click on the 3 dots menu on it's right end. This would open a menu with an option to "Upload ReTrace mapping file (.txt or .map)"

New instructions are listed here, if anyone else is still looking: https://support.google.com/googleplay/android-developer/answer/9848633?hl=en