Fix write start time to execution info (#6590)

## What changed?
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Populate StartTime in mutable state execution info

## Why?
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We cannot delete from there yet: downgrade breaks.

## How did you test it?
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test? Checked in staging env? -->
Start with change, and downgrade temporal works as it should.

## Potential risks
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change to production? -->

## Documentation
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new behavior is added, have you described that in `docs/`? -->

## Is hotfix candidate?
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service/history/workflow/mutable_state_impl.go

@@ -310,6 +310,7 @@ func NewMutableState(
 
 		LastCompletedWorkflowTaskStartedEventId: common.EmptyEventID,
 
+		StartTime:              timestamppb.New(startTime),
 		VersionHistories:       versionhistory.NewVersionHistories(&historyspb.VersionHistory{}),
 		ExecutionStats:         &persistencespb.ExecutionStats{HistorySize: 0},
 		SubStateMachinesByType: make(map[string]*persistencespb.StateMachineMap),