Google now allows you to safely update your app's .jks file in case you have lost it.
In order to be able to do this, you must be enrolled for Google App Signing or give Google the necessary permissions to manage your app signing.
To confirm that you are eligible for this, go to your Google Developer Console then "Release > App Integrity". Just make sure that you have "App Signing" showing "Signing by Google" in place:
After confirming that you are eligible, you can follow the steps from this page to reset your key: https://support.google.com/googleplay/android-developer/answer/9842756#lost