Fix date/time field for GCP logging

[lcobucci]

Fixes #1754

This uses the field that actually maps to the expected format.

More info: https://cloud.google.com/logging/docs/structured-logging

[lcobucci]

@Seldaek it seems like the error comes from the main branch...

[Seldaek]

That's very possible, given the latest PHPStan releases. I'll take a look when I have time :)

[Seldaek]

Thanks