Update default `available_memory_mb` and wording updates to `available_cpu`

Improve wording surrounding available_cpu

https://gitlab.developers.cam.ac.uk/uis/devops/infra/terraform/gcp-scheduled-script/-/blob/master/variables.tf?ref_type=heads#L117

mark available_cup as optional

https://gitlab.developers.cam.ac.uk/uis/devops/infra/terraform/gcp-scheduled-script/-/blob/master/README.md?ref_type=heads&plain=1#L152

and bump the available_memory_mb default to 256.