ORM 3 compat fixes

[mbabker]

This is some low-hanging fruit working as a very first pass for #2708. Best viewed by individual commits.

Three things are tackled:

• The Doctrine\ORM\EntityRepository::$_em property was made private, the getter method is used instead
• The Doctrine\ORM\Internal\Hydration\AbstractHydrator::$_em property was renamed to $em, and it has no getter method; an internal trait was introduced and implemented in all ORM hydrators to support retrieving the entity manager from the parent class in a cross-version compatible manner
• The unit tests will now use the ORM's AttributeDriver instead of the deprecated AnnotationDriver when running on PHP 8 when configuring an entity manager

[codecov]

Codecov Report

Attention: 27 lines in your changes are missing coverage. Please review.

Comparison is base (4a0d0f9) 79.67% compared to head (b76536d) 79.68%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2709   +/-   ##
=======================================
  Coverage   79.67%   79.68%           
=======================================
  Files         161      162    +1     
  Lines        8453     8455    +2     
=======================================
+ Hits         6735     6737    +2     
  Misses       1718     1718           

Files	Coverage Δ
.../Loggable/Entity/Repository/LogEntryRepository.php	96.96% <100.00%> (ø)
.../Sortable/Entity/Repository/SortableRepository.php	91.17% <100.00%> (ø)
src/Tool/ORM/Hydration/EntityManagerRetriever.php	100.00% <100.00%> (ø)
src/Translatable/Hydrator/ORM/ObjectHydrator.php	93.33% <100.00%> (ø)
...Translatable/Hydrator/ORM/SimpleObjectHydrator.php	93.33% <100.00%> (ø)
.../Tree/Entity/Repository/AbstractTreeRepository.php	80.85% <100.00%> (ø)
...e/Entity/Repository/MaterializedPathRepository.php	98.36% <100.00%> (ø)
...atable/Entity/Repository/TranslationRepository.php	84.07% <93.33%> (ø)
src/Tree/Hydrator/ORM/TreeObjectHydrator.php	88.37% <80.00%> (ø)
...rc/Tree/Entity/Repository/NestedTreeRepository.php	89.86% <94.11%> (ø)
... and 1 more

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[franmomu]

thanks @mbabker!