Fix pydantic 2.2.1 RootModel cannot have extra_fields error

[i404788]

Since Pydantic 2.2.1, it will through an exception if extra_fields are defined for a RootModel:

  File "/home/null/mambaforge/lib/python3.10/site-packages/pydantic/_internal/_model_construction.py", line 123, in __new__
    cls: type[BaseModel] = super().__new__(mcs, cls_name, bases, namespace, **kwargs)  # type: ignore
  File "/home/null/mambaforge/lib/python3.10/abc.py", line 106, in __new__
    cls = super().__new__(mcls, name, bases, namespace, **kwargs)
  File "/home/null/mambaforge/lib/python3.10/site-packages/pydantic/root_model.py", line 50, in __init_subclass__
    raise PydanticUserError(
pydantic.errors.PydanticUserError: `RootModel` does not support setting `model_config['extra']`

The following code will remove that option if --allow-extra-fields or similar behavior is enabled

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (e9b6edf) 100.00% compared to head (822083e) 100.00%.
Report is 704 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #1497     +/-   ##
===========================================
  Coverage   100.00%   100.00%             
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  Files           11        32     +21     
  Lines         1020      3589   +2569     
  Branches       201       840    +639     
===========================================
+ Hits          1020      3589   +2569     

Flag	Coverage Δ
unittests	99.63% <99.58%> (?)

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Files Changed	Coverage Δ
datamodel_code_generator/__init__.py	100.00% <100.00%> (ø)
datamodel_code_generator/__main__.py	100.00% <100.00%> (ø)
datamodel_code_generator/format.py	100.00% <100.00%> (ø)
datamodel_code_generator/http.py	100.00% <100.00%> (ø)
datamodel_code_generator/imports.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/__init__.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/base.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/dataclass.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/enum.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/imports.py	100.00% <100.00%> (ø)
... and 22 more

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