Upgrade Bazel CI machines (#2689) (#2862)

Migrate Bazel CI runner fleet to C3D instances and scale pools

- Upgrades CI runner pools from `c2-standard-30` to `c3d-standard-30`
and scales the untrusted fleet sizes to improve build speed and queue
throughput.
- Enables Google Virtual NIC (`GVNIC`) support on all custom runner
images. This is mandatory for GCP 3rd-generation machine types (like
C3D) because they do not support legacy VirtIO networking.
- Upgrades the base image builder VM to `c3d-standard-8`.