Reference: https://github.com/hashicorp/terraform/issues/30373
This change forward ports the `legacy_type_system` boolean fields in the `ApplyResourceChange.Response` and `PlanResourceChange.Response` messages that existed in protocol version 5, so that existing terraform-plugin-sdk/v2 providers can be muxed with protocol version 6 providers (e.g. terraform-plugin-framework) while also taking advantage of the newer protocol features. This functionality should not be used by any providers or SDKs except those built with terraform-plugin-sdk.
Updated via:
```shell
cp docs/plugin-protocol/tfplugin6.1.proto docs/plugin-protocol/tfplugin6.2.proto
# Copy legacy_type_system fields from tfplugin5.2.proto into ApplyResourceChange.Response and PlanResourceChange
rm internal/tfplugin6/tfplugin6.proto
ln -s ../../docs/plugin-protocol/tfplugin6.2.proto internal/tfplugin6/tfplugin6.proto
go run tools/protobuf-compile/protobuf-compile.go `pwd`
# Updates to internal/plugin6/grpc_provider.go
```
Brian Flad