chore(deps): update dependency google/protobuf.dart to v21

[renovate]

This PR contains the following updates:

Package	Update	Change
google/protobuf.dart	major	v20.0.1 -> v21.0.1

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Release Notes

google/protobuf.dart

v21.0.1: protoc_plugin-21.0.1

Compare Source

• Fix missing protobuf import in generated grpc files. (#​844)

v21.0.0: protoc_plugin-21.0.0

Compare Source

• Identifiers fromBuffer, fromJson, $_defaultFor, initByValue are no longer reserved. Proto fields with those Dart names will no longer have a suffix added. (#​679)

• Remove message constructor arguments. Constructors with arguments cause increase in release binary sizes even when no arguments are passed to the constructors. (#​703)

  Migration:

  Set the fields after construction, using cascade syntax. For example, if you have:

  MyMessage(a: 123, b: [1, 2, 3])

  You can do:

  MyMessage()
    ..a = 123
    ..b.addAll([1, 2, 3])

• Require Dart 2.19.

• Export public dependencies (import publics in proto files) in .pbenum.dart files, same as .pb.dart files. (9aad6aa)

• Fix decoding map fields when key or value (or both) fields of a map entry is missing. (#​719, #​745)

• Generated files now split ignore_for_file comments across multiple lines when necessary. (#​770)

• Generated files now uses shared consts to eliminate repeated bool.fromEnvironment() expressions. (#​772)

• Removed accidental /// at the top of generated Dart files to avoid new dangling_library_doc_comments lint. (#​774)

• Generated files now have sorted imports and have fewer import-related ignore_for_file: analysis directives. (#​778)

• Remove duplicated consts in generated files. (#​773)

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