Bugfix: Use specific entity prefixes for key generation when duplicating entities

[priethor]

What

Fixes a bug where duplicated entities would always have the group_ prefix instead of the prefix of the entity.

Why

It's inconsistent with the keys of new entities, and won't pass schema validations.

How

In the base class ACF_Internal_Post_Type, use the attribute post_key_prefix, which child classes override, instead of hardcoding the prefix.

Testing instructions

1. Create a post type
2. Duplicate it
3. Check that its internal key doesn't start with group_ anymore, but with post_type_

[github-actions]

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Props priethor, cbravobernal.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 3.37%. Comparing base (e870b55) to head (1149b6b).
⚠️ Report is 2 commits behind head on trunk.

Additional details and impacted files

@@            Coverage Diff             @@
##             trunk    #240      +/-   ##
==========================================
+ Coverage     1.17%   3.37%   +2.19%     
  Complexity    4416    4416              
==========================================
  Files          261     261              
  Lines        27017   27017              
==========================================
+ Hits           318     911     +593     
+ Misses       26699   26106     -593     

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