Fix task priority name (temporalio#7952)

## What changed?
- Fix name for task priority preemptable

## Why?
- Without the fix the logic won't be able to parse task weight configs and
causing errors "Unknown key for task priority name" and always use the
default task priority weight.

## How did you test it?
- [ ] built
- [ ] run locally and tested manually
- [x] covered by existing tests
- [ ] added new unit test(s)
- [ ] added new functional test(s)

Author: yycptt

common/tasks/priority.go

@@ -40,9 +40,9 @@ var (
 	}
 
 	PriorityValue = map[string]Priority{
-		"high":       PriorityHigh,
-		"low":        PriorityLow,
-		"background": PriorityPreemptable,
+		"high":        PriorityHigh,
+		"low":         PriorityLow,
+		"preemptable": PriorityPreemptable,
 	}
 
 	CallerTypeToPriority = map[string]Priority{