[ENHANCEMENT] Enable Multiplex Spanner Sessions Successfully

We have been [excluding](https://github.com/GoogleChrome/webstatus.dev/blob/7a6ffd51e418cceb4e3486e6d5865074715e1f9a/lib/go.mod#L183-L190) the newer version of the spanner SDK because it broke the tests.

It is because of the fact that Multiplex Sessions became [enabled by default](https://cloud.google.com/spanner/docs/sessions#enable-multiplex) in the newer versions of the SDK (starting with 1.85.0) and the emulator doesn't seem to handle it well.

For now, we disable it so that it acts like prior to 1.85.0.

We should revisit it and re-enable it by removing the lines in client.go that set `GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS` and `GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_FOR_RW` to true and running the tests again.