Bump protobuf dependency to 4.0.0 and prepare to release 4.0.3

[mraleph]

No description provided.

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:grpc	4.0.3	ready to publish	v4.0.3

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
grpc	Non-Breaking	4.0.2	4.0.3	4.0.3	✔️

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

Coverage ✔️

File	Coverage

This check for test coverage is informational (issues shown here will not fail the PR).

API leaks ⚠️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbols
grpc	Any $1.Duration

This check can be disabled by tagging the PR with skip-leaking-check.

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
grpc	Breaking	4.0.2	5.0.0	5.0.0	✔️

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

Coverage ⚠️

File	Coverage
lib/src/generated/google/protobuf/any.pb.dart	💚 28 %
lib/src/generated/google/protobuf/any.pbenum.dart	💔 Not covered
lib/src/generated/google/protobuf/any.pbjson.dart	💔 Not covered
lib/src/generated/google/protobuf/duration.pb.dart	💔 0 % ⬇️ NaN %
lib/src/generated/google/protobuf/duration.pbenum.dart	💔 Not covered
lib/src/generated/google/protobuf/duration.pbjson.dart	💔 Not covered
lib/src/generated/google/rpc/error_details.pb.dart	💚 4 %
lib/src/generated/google/rpc/error_details.pbenum.dart	💔 Not covered
lib/src/generated/google/rpc/error_details.pbjson.dart	💔 Not covered
lib/src/generated/google/rpc/status.pb.dart	💚 30 %
lib/src/generated/google/rpc/status.pbenum.dart	💔 Not covered
lib/src/generated/google/rpc/status.pbjson.dart	💔 Not covered

This check for test coverage is informational (issues shown here will not fail the PR).

This check can be disabled by tagging the PR with skip-coverage-check.

API leaks ⚠️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbols
grpc	Any $1.Duration

This check can be disabled by tagging the PR with skip-leaking-check.

[mraleph]

@mkustermann could you rubberstamp?