Update project policy to use admin_push_rules

What does this MR do and why?

• Applies the custom role ability admin_push_rules to project policy, so that when a user is assigned this ability they can configure push rule settings at the project level.
• Follows from !147872 (merged) and !147974 (merged)

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.

Screenshots or screen recordings

How to set up and validate locally

1. Enable the custom_ability_admin_push_rules Feature Flag:

   Feature.enable(:custom_ability_admin_push_rules)

2. Ensure SaaS mode is on with

export GITLAB_SIMULATE_SAAS=1
gdk restart

3. Log-in as an owner of a group and navigate to Settings > Roles and Permissions
4. Create a custom role with Admin push rules checkbox enabled.
5. Navigate to the group's Manage > Members and assign this custom role to a user
6. Log-in as that user and navigate to a project inside that group. You should see Settings > Repository where you can see a section for Push rules.

Related to #421786 (closed)