chore(deps): update built_value to ^8.12.6

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
built_value (source)	dev_dependencies	minor	^8.10.1 → ^8.12.6
built_value_generator (source)	dev_dependencies	minor	^8.10.1 → ^8.12.6
built_value_test (source)	dev_dependencies	minor	^8.10.1 → ^8.12.6

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Release Notes

google/built_value.dart (built_value)

v8.12.6

Compare Source

• Require analyzer 13.0.0.

v8.12.5

Compare Source

• Allow analyzer 11.0.0 and analyzer 12.0.0.

v8.12.4

Compare Source

• Reduce use of dynamic calls in generated operator== when there are
  functions: only use dynamic calls if the class also has type parameters.

v8.12.3

Compare Source

• Require analyzer ^10.0.0, stop using removed and deprecated methods from
  earlier versions.

v8.12.2

Compare Source

• Allow analyzer 10.0.0.

v8.12.1

• Allow analyzer 9.0.0.

v8.12.0

• Enable build_runner 2.7.0 "triggers" optimization.
• built_value will now only run in libraries that directly import
  package:built_value/built_value.dart or use an annotation called
  @SerializersFor. Other files will be quickly skipped and show in
  build_runner output as "not triggered" or "skipped".
• If you need to run built_value on other source files, see
  https://pub.dev/packages/build_config#triggers for how to configure your own
  additional triggers for built_value.

v8.11.2

• Require analyzer ^8.0.0.
• Allow built_value_generator to use build 4.0.0.
• Allow built_value_generator to use source_gen 4.0.0.

v8.11.1

• Allow built_value_generator to use analyzer 8.0.0.

v8.11.0

Compare Source

• Use build ^3.0.0.
• Use source_gen ^3.0.0.

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