Switch arm64 instance groups to c4a-standard-8 (#2807)

Switch ARM64 instance groups from `c4a-standard-8-lssd` to standard
`c4a-standard-8`.

The instances already use `hyperdisk-balanced` with
`startup-docker-pdssd.sh` and do not mount or format the attached Local
SSD. Using the `-lssd` variant causes zone capacity exhaustion
(`ZONE_RESOURCE_POOL_EXHAUSTED`) in `us-central1-c` and local SSD quota
exhaustion.

Also increased the testing machines to 10