Use go 1.16

[wxing1292]

No description provided.

[andrewcohen]

For what it's worth, I run into a handful of build errors when using this branch with go 1.16. Would these errors go away once the package is published to go.temporal.io?

❯ go version    
go version go1.16 darwin/amd64

❯ go build ./...
../../../go/src/github.com/temporalio/sdk-go/internal/internal_decision_state_machine.go:32:2: missing go.sum entry for module providing package go.temporal.io/api/command/v1 (imported by go.temporal.io/sdk/internal); to add:
        go get go.temporal.io/sdk/internal@v1.4.1
../../../go/src/github.com/temporalio/sdk-go/client/client.go:34:2: missing go.sum entry for module providing package go.temporal.io/api/common/v1 (imported by go.temporal.io/sdk/client); to add:
        go get go.temporal.io/sdk/client@v1.4.1
../../../go/src/github.com/temporalio/sdk-go/client/client.go:35:2: missing go.sum entry for module providing package go.temporal.io/api/enums/v1 (imported by go.temporal.io/sdk/client); to add:
        go get go.temporal.io/sdk/client@v1.4.1
../../../go/src/github.com/temporalio/sdk-go/internal/error.go:35:2: missing go.sum entry for module providing package go.temporal.io/api/failure/v1 (imported by go.temporal.io/sdk/internal); to add:
        go get go.temporal.io/sdk/internal@v1.4.1
../../../go/src/github.com/temporalio/sdk-go/internal/internal_decision_state_machine.go:36:2: missing go.sum entry for module providing package go.temporal.io/api/history/v1 (imported by go.temporal.io/sdk/worker); to add:
        go get go.temporal.io/sdk/worker@v1.4.1
../../../go/src/github.com/temporalio/sdk-go/internal/internal_task_handlers.go:46:2: missing go.sum entry for module providing package go.temporal.io/api/query/v1 (imported by go.temporal.io/sdk/internal); to add:
        go get go.temporal.io/sdk/internal@v1.4.1
../../../go/src/github.com/temporalio/sdk-go/internal/grpc_dialer.go:33:2: missing go.sum entry for module providing package go.temporal.io/api/serviceerror (imported by go.temporal.io/sdk/internal); to add:
        go get go.temporal.io/sdk/internal@v1.4.1
../../../go/src/github.com/temporalio/sdk-go/internal/internal_event_handlers.go:44:2: missing go.sum entry for module providing package go.temporal.io/api/taskqueue/v1 (imported by go.temporal.io/sdk/internal); to add:
        go get go.temporal.io/sdk/internal@v1.4.1
../../../go/src/github.com/temporalio/sdk-go/client/client.go:36:2: missing go.sum entry for module providing package go.temporal.io/api/workflowservice/v1 (imported by go.temporal.io/sdk/client); to add:
        go get go.temporal.io/sdk/client@v1.4.1
../../../go/src/github.com/temporalio/sdk-go/internal/internal_worker.go:56:2: missing go.sum entry for module providing package go.temporal.io/api/workflowservicemock/v1 (imported by go.temporal.io/sdk/internal); to add:
        go get go.temporal.io/sdk/internal@v1.4.1
        ```

[wxing1292]

@andrewcohen this PR aims to update the master branch of go SDK, there will be release afterwards. before the release, we will upgrade our internal package for verification.