Update code generator to pass lints-2.0.0 recommended lints

[osa1]

lints-2.0.0 enables a few new lints, and currently code we generate does not follow no_leading_underscores_for_local_identifiers as we generate _result in message factory methods.

Fix should be as easy as renaming the variable to $_result. We can't rename it to anything that could be a Dart identifier for a proto field (e.g. result) as that will cause shadowing in the method body.

We should also update protobuf and protoc_plugin lints deps to 2.0. If we do that then we will start checking generated files for lints-2.0 as we run dart analyze after generating protos in CI.

[osa1]

With #703 we no longer generate the _result variable and generated files pass lints-2.0.0 recommended lints.

See protoc_plugin changelog for how to migrate your code for #703, which is a breaking change.