Default access_as in ci_access and user_access

We should have sensible access_as default values for ci_access and user_access. This has only to be configured in the agent_configuration module.

Update the docs at: https://docs.gitlab.com/user/clusters/agent/user_access/#:~:text=access_as%3A%20Required.%20For%20plain%20access%2C%20the%20value%20is%20%7B%20agent%3A%20%7B...%7D%20%7D