Update dependency protobuf to v6

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
protobuf (source)	dependencies	major	^3.1.0 → ^6.0.0

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Release Notes

google/protobuf.dart (protobuf)

v6.1.0

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• Make BuilderInfo methods accept GeneratedMessage Function() typed
  closures.
• Make BuilderInfo.add accept nullable name (for tree shaking purposes).
• Remove use of legacy dart.library.html conditional import.

v6.0.0

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• New GeneratedMessage extension methods toTextFormat and writeTextFormat
  added to convert the message into the official protocol buffers text
  format. (#​1080, #​125)

• Add well-known proto types as libraries. This change is required for
  protoc_plugin-25.0.0. (#​1081)

• Breaking: Hide PbList and PbMap constructors. It is not possible to
  construct these values correctly in user code, so the constructors are now
  private. Existing uses of PbList can be replaced by List and PbMap can
  be replaced by Map.

  For immutable lists and maps, you can use built_value. (#​1072)

• Map fields now check key and value validity when adding elements. (#​1065,
  #​1076)

v5.1.0

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• Update default size limit of CodedBufferReader from 67,108,864 bytes to
  2,147,483,647 bytes, and default recursion limit from 64 to 100.

  The new limits are consistent with the Java and C++ implementations. (#​1060)

• Fix GeneratedMessage.addExtension returning non-frozen and
  GeneratedMessage.getExtension allowing modifying an extension when the
  message is frozen before initializing the extension field set. (#​1062)

• Fix GeneratedMessage.getExtension returning differently typed lists when the
  message extension field set is initialized and frozen and initialized but not
  frozen. (#​1062)

• Fix PbList methods addAll, insertAll, replaceRange, setAll,
  setRange iterating the Iterable argument twice. (#​730, #​1070)

• Fix GeneratedMessage.== throwing a type error when comparing map fields in
  some cases. (#​1075, #​1077)

  This bug was introduced with protobuf-5.0.0.

v5.0.0

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• Improve performance of GeneratedMessage.deepCopy. (#​742)
• Fix unknown enum handling in GeneratedMessage.mergeFromProto3Json when
  the ignoreUnknownFields optional argument is true. (#​853)
• Add BuilderInfo methods to support protoc-plugin 23.0.0. (#​1047)
• Generalize argument type of PbList.from from List<T> to Iterable<T>.
  (#​1054)
• Fix clearing oneof fields with GeneratedMessage.clear. (#​1057)
• Fix unknown JSON handling when using GeneratedMessage methods
  mergeFromJson, mergeFromJsonMap, writeToJson, writeToJsonMap.
  (#​1058)

v4.2.0

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• Internal refactoring to split the package into libraries. This allows
  conditionally importing different libraries and improving performance by
  using different encoding/decoding libraries based on the target platform.
  (#​1026)

• Some of the private PbFieldType members are made public, to allow using
  them in internal libraries. This type is for internal use only. (#​1027)

• Improve performance of GeneratedMessage members: writeToJsonMap,
  writeToJson, mergeFromJson, mergeFromJsonMap. (#​1028)

• Remove BuilderInfo.fromProto3Json and BuilderInfo.toProto3Json as a part
  of an internal refactoring.

v4.1.1

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• Minimum SDK dependency bumped from 3.6.0 to 3.7.0. (#​1024)

v4.1.0

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• Improve packed field decoding performance. (#​959, #​981)
• Minimum SDK dependency bumped from 3.3.0 to 3.6.0. (#​1001)

v4.0.0

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• Breaking: The following types and members are now removed:

  • PbEventMixin
  • PbFieldChange
  • EventBuffer
  • GeneratedMessage.createRepeatedField
  • GeneratedMessage.createMapField

  These were used to implement events, which are unused internally. To keep API
  surface small (to make it easier to change the library or migrate to another
  library) these types and members are removed. (#​738)

• Breaking: CodedBufferWriter.writeRawBytes now takes a Uint8List
  argument (instead of TypedData).

• GeneratedMessageGenericExtensions.deepCopy is now annotated with
  @useResult and will generate a warning when its result is not used.
  (#​896)

• Breaking: PbMap.unmodifiable now takes key and value field types as
  arguments, instead of another PbMap.

  To migrate, use PbMap.unmodifiable(map.keyFieldType, map.valueFieldType)
  instead of PbMap.unmodifiable(map). (#​902)

• Messages deserialized from JSON now generate the unknown fields when
  serialized as JSON.

  Note that, as before, unknown fields in JSON messages are not stored in the
  unknownFields of the message. They are only used by the JSON serializers to
  support roundtripping.

  (#​49, #​918)

• Minimum SDK dependency bumped from 2.19.0 to 3.3.0. (#​953)

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