Alerting: Add clean_upgrade config and deprecate force_migration (#78324)

* Alerting: Add clean_upgrade config and deprecate force_migration

Upgrading to UA and rolling back will no longer delete any data by default. 
Instead, each set of tables will remain unchanged when switching between 
legacy and UA. As such, the force_migration config has been deprecated 
and no extra configuration is required to roll back to legacy anymore.

If clean_upgrade is set to true when upgrading from legacy alerting to Unified
Alerting, grafana will first delete all existing Unified Alerting resources,
thus re-upgrading all organizations from scratch. If false or unset,
organizations that have previously upgraded will not lose their existing Unified
 Alerting data when switching between legacy and Unified Alerting.

 Similar to force_migration, it should be kept false when not needed as it may
 cause unintended data-loss if left enabled.

---------

Co-authored-by: Christopher Moyer <35463610+chri2547@users.noreply.github.com>

pkg/setting/setting.go

@@ -416,6 +416,7 @@ type Cfg struct {
 	StackID string
 	Slug    string
 
+	// Deprecated
 	ForceMigration bool
 
 	// Analytics
@@ -1061,6 +1062,7 @@ func (cfg *Cfg) Load(args CommandLineArgs) error {
 	cfg.Env = Env
 	cfg.StackID = valueAsString(iniFile.Section("environment"), "stack_id", "")
 	cfg.Slug = valueAsString(iniFile.Section("environment"), "stack_slug", "")
+	//nolint:staticcheck
 	cfg.ForceMigration = iniFile.Section("").Key("force_migration").MustBool(false)
 	InstanceName = valueAsString(iniFile.Section(""), "instance_name", "unknown_instance_name")
 	plugins := valueAsString(iniFile.Section("paths"), "plugins", "")