Update the scaling policies when deregistering a job (#25911)

* func: Update the scaling policies when deregistering a job

* func: Add tests for updating the policy

* docs: add changelog

* func: set back the old order

* style: rearrange for clarity and to reuse the watchset

* func: set the policies to teh last submitted when starting a job

* func: expand tests  of teh start job command to include job submission

* func: Expand the tests to verify the correct state of the scaling policy after job start

* Update command/job_start.go

Co-authored-by: Tim Gross <tgross@hashicorp.com>

* Update nomad/fsm_test.go

Co-authored-by: Tim Gross <tgross@hashicorp.com>

* func: add warning when there is no previous job submission

---------

Co-authored-by: Tim Gross <tgross@hashicorp.com>

api/jobs.go

@@ -1420,6 +1420,15 @@ func (j *Job) AddSpread(s *Spread) *Job {
 	return j
 }
 
+func (j *Job) GetScalingPoliciesPerTaskGroup() map[string]*ScalingPolicy {
+	ret := map[string]*ScalingPolicy{}
+	for _, tg := range j.TaskGroups {
+		ret[*tg.Name] = tg.Scaling
+	}
+
+	return ret
+}
+
 type WriteRequest struct {
 	// The target region for this write
 	Region string

api/tasks.go

@@ -707,6 +707,12 @@ func (g *TaskGroup) AddSpread(s *Spread) *TaskGroup {
 	return g
 }
 
+// AddSpread is used to add a new spread preference to a task group.
+func (g *TaskGroup) ScalingPolicy(sp *ScalingPolicy) *TaskGroup {
+	g.Scaling = sp
+	return g
+}
+
 // LogConfig provides configuration for log rotation
 type LogConfig struct {
 	MaxFiles      *int `mapstructure:"max_files" hcl:"max_files,optional"`