Change the placeholder for the sluggable extension when generating a slug for an identifier field

[mbabker]

Ref: #2708

In doctrine/orm#10785 (shipped in ORM 2.16.0) a check was added to the UnitOfWork to prevent ID collisions in its internal map, which was then changed to a deprecation in doctrine/orm#10878 (shipped in ORM 2.16.1) due to its disruptiveness. That check is back in 3.0.

This PR avoids the issue by changing the ID placeholder used by the sluggable extension when it is configured to generate slugs for an identifier field to a placeholder based on uniqid() and fixes the last PHPUnit error on my hack-it-together-ORM-3-support branch.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.91%. Comparing base (0632ab1) to head (6076c5a).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2768      +/-   ##
==========================================
+ Coverage   78.75%   78.91%   +0.16%     
==========================================
  Files         163      163              
  Lines        8593     8502      -91     
==========================================
- Hits         6767     6709      -58     
+ Misses       1826     1793      -33     

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[franmomu]

Thanks @mbabker!