Update dependency protobuf to v3

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
protobuf	dependencies	major	^2.0.1 -> ^3.0.0

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Release Notes

google/protobuf.dart (protobuf)

v3.1.0

Compare Source

• CodedBufferReader readBytes now copies the returned bytes to avoid
  accidental sharing of the input buffer with the returned Uint8List. New
  member readBytesAsView added with the old behavior. (#​863)

• Avoid sharing the input buffer in unknown length-delimited fields using the
  new readBytes. (#​863)

v3.0.0

Compare Source

• Require Dart 2.19.
• Remove ReadonlyMessageMixin (#​183, #​644)
• Remove frozenMessageModificationHandler (#​175, #​643)
• Remove PbListBase and FrozenPbList types. All proto repeated fields now
  use PbList. To check if a list is frozen, use isFrozen getter. (#​624,
  #​626)
• Initialize map fields in GeneratedMessage.getField. This behavior is
  consistent with getField called on repeated fields. (#​373, #​707)
• Remove unused and optional PbMap constructor argument BuilderInfo? info.
  (d94d3f0)
• UnknownFieldSetField methods hasRequiredFields, isInitialized and
  getter length removed. (#​721)
• Update library documentation to hide internals, add documentation for public
  types. (#​681)
• Fix PbMap._isReadonly field initialization in PbMap.unmodifiable.
  (#​741, #​754)
• Fix decoding map fields when key or value (or both) fields of a map entry is
  missing. (#​719, #​745)
• Fix updating frozen (immutable) messages with merge methods
  (mergeFromBuffer, mergeFromProto3Json, ...). (#​489, #​727)
• Fix handling null values in proto3 JSON deserializer. (#​751, #​760,
  #​763)
• Fix handling negative JSON values when parsing uint32 fields. (#​839)
• Avoid serializing unknown fields twice in reparseMessage. (#​840)

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