fix(deps): update dependency protobuf to v6

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
protobuf (source)	dependencies	major	^4.2.0 → ^6.0.0

⚠️ MAJOR MAJOR MAJOR ⚠️

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Release Notes

google/protobuf.dart (protobuf)

v6.0.0

Compare Source

• 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

Compare Source

• 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

Compare Source

• 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)

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