Update proto file definition of Read/WriteStateBytes RPCs

[SarahFrench]

Updates the base branch to include proto changes introduced in hashicorp/terraform-plugin-go#531

Target Release

N/A

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

[SarahFrench]

In f705ad7 I was fixing this test failure:

--- FAIL: TestGRPCProvider_ReadStateBytes (0.00s)
    --- FAIL: TestGRPCProvider_ReadStateBytes/warning_diagnostics_from_the_provider_are_returned (0.00s)
        grpc_provider.go:1516: Unexpected call to *mock_tfplugin6.MockProvider_ReadStateBytesClient.Recv([]) at /home/runner/work/terraform/terraform/internal/plugin6/grpc_provider.go:1516 because: 
            expected call at /home/runner/work/terraform/terraform/internal/plugin6/grpc_provider_test.go:3755 has already been called the max number of times
FAIL
coverage: 60.5% of statements

...and I found that diagnostics returned with the EOF were not being passed through.

I think this was missed when merging tests into the base branch in the past? Not sure.