Choose sizing properties for redis-feature-flag

We need to decide on machine-type and resource-related Redis settings. redis-feature-flag is a subset of redis-cache.

For reference:

• redis-cache uses a c2-standard-60 (https://ops.gitlab.net/gitlab-com/gl-infra/config-mgmt/-/blob/master/environments/gprd/variables.tf#L479).
• redis (shared-state) uses c2-standard-16