Add default config for redis.repository_cache.yml

What does this merge request do and why?

As part of adding a new Redis instance for Repository Cache, we're also adding the default config for redis.repository_cache.yml. The default configuration follows the redis.cache.yml instance.

How to set up and validate locally

1. Ensure gitlab/config/redis.repository_cache.yml is not present
2. Run gdk reconfigure
3. Check the generated file, should match with gitlab/config/redis.cache.yml

$ cat gitlab/config/redis.repository_cache.yml                                                                                                                                                                                                                                                                                                                                            
---
development: unix:/Users/gregoriusmarco/Documents/workspace/gdk-10-22/redis/redis.socket?db=2
test: unix:/Users/gregoriusmarco/Documents/workspace/gdk-10-22/redis/redis.socket?db=12

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.