Relax grpc version pin to 1.3x

Pinning this at the patch level makes it impossible to experiment with grpc upgrades in repositories that depend on labkit.

We recently had to revert grpc to an older version due to issues on older hardware. However, that older version is incompatible with Ruby 3, blocking our progress on it.

To at least have a green build in our experimental ruby-3 branch, it would be useful for that branch to use a newer grpc version than on master, but this requires a less strict pin in labkit, which itself is a dependency on the main app.