The repository ' stable Release' is not signed

You don't. You must wait for Google to renew their keys and for an update.

The important message is:

The following signatures were invalid: EXPKEYSIG 1397BC53640DB551 Google Inc. (Linux Packages Signing Authority)

It means that the cryptographic signature is invalid. The source of this can be an attack, a misconfiguration, or other kind of technical problem. Forcing your system to update will result in running an unverified version of your web browser, which can expose you to a lot of security troubles.

It's the same as this question (10 mins of difference):

Short explanation: the GPG key on Google side expired, so you (we) have to wait.

Plan 1

This is the protection you are getting from these checks. You don't want to update your software right now while something is messed up on Google's end. Wait until they fix it. Don't try to override by reinstalling keys until some official word comes out that a new key is the solution.

Plan 2

Waiting until they fix it may not be an option for all. E.g. this is breaking CI pipelines for us. If you now what you are doing, you might take the risk and disable checks for this repo for now by adding [trusted=yes] to it's configuration: deb [trusted=yes] stable main –