Reduce deprecations due to FieldMapping array access

[Jean85]

Fixes #2827, at least in large part.

I did a simple regex find&replace of access to $mapping['...'], and then:

• added parentheses everywhere, since ?? (null coalescing) has one of the lowest operator precedence in PHP
• removed the parentheses (with another regex) when:
  • it was already inside a pair of them
  • it was an argument (so already separated by parentheses and\or a comma)
• reviewed all the changes manually and:
  • reverted the changes in the ODM class (since there is no deprecation there)
  • reverted all the times where it was inside an isset call (it wouldn't work)
  • removed the parentheses manually when it was already followed by a ?? (so it's a concatenation)

I realize this may not cover all the places where this deprecation may be triggered, but I'm sure that all my changes are covering some, so it's a good start.

[Jean85]

Ok I renamed my first commit (since it addressed only variables named $mapping) and added a second one for $fieldMapping. I'll proceed to add more stuff.

[Jean85]

Ok that's all for me. Let me know if there are more ways to fetch the FieldMapping object that I should inspect.

[mbabker]

Check the changes in the YAML mapping drivers, some of those are parsing the YAML files as arrays and not from the metadata objects.

[mbabker]

Likewise, anything that's in MongoDB ODM specific code (like document repository classes) doesn't need to change as the deprecations only affect the ORM.

[Jean85]

@mbabker I addressed your comments with some reverts; it may be a good idea to squash this PR while merging.

[Jean85]

Thanks @mbabker, I've addressed all the comments of your review.

[codecov]

Codecov Report

Attention: Patch coverage is 84.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 78.67%. Comparing base (ee63c20) to head (4a89cd3).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
.../ReferenceIntegrity/ReferenceIntegrityListener.php	75.00%	3 Missing ⚠️
src/Blameable/Mapping/Driver/Xml.php	0.00%	1 Missing ⚠️
src/Blameable/Mapping/Driver/Yaml.php	0.00%	1 Missing ⚠️
src/IpTraceable/Mapping/Driver/Xml.php	0.00%	1 Missing ⚠️
src/IpTraceable/Mapping/Driver/Yaml.php	0.00%	1 Missing ⚠️
src/SoftDeleteable/Mapping/Validator.php	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2889      +/-   ##
==========================================
- Coverage   78.82%   78.67%   -0.15%     
==========================================
  Files         167      167              
  Lines        8637     8752     +115     
==========================================
+ Hits         6808     6886      +78     
- Misses       1829     1866      +37     

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