chore: update dependency source_gen to v2

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
source_gen (source)	dependencies	major	^1.5.0 -> ^2.0.0

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Release Notes

dart-lang/source_gen (source_gen)

v2.0.0

Compare Source

• Breaking Change: Change formatOutput function to accept a language
  version parameter.
• Formatting Change: Generated code will no longer apply any fixes by
  default (previously it would apply the single cascades statements fix). The
  new formatter does not support applying fixes.
• Document deduplication behavior for the output of
  GeneratorForAnnotation.generateForAnnotatedElement.
• Support all the glob quotes.
• Require analyzer: '>=6.9.0 <8.0.0'
• Support the latest package:dart_style
• LibraryBuilder, PartBuilder, and SharedPartBuilder now take an optional
  writeDescriptions boolean. When set to false, headers and generator
  descriptions for the files will not be included in the builder output.
• Include //dart format width=80 comments in files generated by a
  LibraryBuilder or PartBuilder and formatted with the default callback.
• Require Dart 3.6.0

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