Update gRPC to 1.64.0 #1708

I'm not sure how to make a pull request for this, as it seems to be defined in the maven artifact first-party-dependencies: https://central.sonatype.com/artifact/com.google.cloud/first-party-dependencies

We've seen a lot of "internal errors" lately downstream, in Flutter, which it's my understanding stems from gRPC.

So here's a request to update to the latest version of gRPC, which as of writing is 1.64.0.

The exact error: [cloud_firestore/internal] Internal errors. Means some invariants expected by underlying system has been broken. If you see one of these errors, something is very broken.