Move 'ServiceAccessToken' from Ai to CC namespace

What does this MR do and why?

Refactor the Ai::ServiceAccessToken model to become CloudConnector::ServiceAccessToken.

This class serves a wider purpose than just AI.
The goal is to group all CloudConnector-related things under the dedicated namespace.

Safety

🗒 Even though we rename the model class, it explicitly refers the DB table name (code) which is unchanged.
So, I understand this is a safe change (no downtime expected).
Please ping me if you have any concerns; I am happy to discuss them!

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

Fire up the rails console and check that the new model name works (no undefined-const-related issues):

[1] pry(main)> CloudConnector::ServiceAccessToken.count
  CloudConnector::ServiceAccessToken Count (0.2ms)  SELECT COUNT(*) FROM "service_access_tokens" /*application:console,db_config_name:main,console_hostname:Alekseis-MBP-2,console_username:al,line:(pry):1:in `__pry__'*/
=> 2

Related to #438680 (closed)