Use a more specific policy for new approval rule model

What does this MR do and why?

Use a more specific policy for new approval rule model

Add a new policy to add the new approval rule model to be used for project level approval rules.

Since MergeRequests::ApprovalRule can replace ApprovalProjectRule and ApprovalMergeRequestRule depending on what the origin this new policy should behave similarly to the corresponding legacy policies.

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Before	After

How to set up and validate locally

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Related to #519595 (closed)